From f4705246a258feaf6fb95bff0d2b2be6096f850b Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Mateusz=20Burzy=C5=84ski?= <mateuszburzynski@gmail.com>
Date: Fri, 17 Oct 2025 23:35:27 +0200
Subject: [PATCH 1/6] Fixed crash when using intersection with private
 constructors as a base class

---
 src/compiler/checker.ts                       |  20 ++-
 .../extendPrivateConstructorClass2.errors.txt |  71 ++++++++
 .../extendPrivateConstructorClass2.symbols    | 104 ++++++++++++
 .../extendPrivateConstructorClass2.types      | 153 ++++++++++++++++++
 .../extendPrivateConstructorClass2.ts         |  49 ++++++
 5 files changed, 389 insertions(+), 8 deletions(-)
 create mode 100644 tests/baselines/reference/extendPrivateConstructorClass2.errors.txt
 create mode 100644 tests/baselines/reference/extendPrivateConstructorClass2.symbols
 create mode 100644 tests/baselines/reference/extendPrivateConstructorClass2.types
 create mode 100644 tests/cases/compiler/extendPrivateConstructorClass2.ts

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 48bc0da113816..582cdc04e3108 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -28402,6 +28402,10 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
         return type.flags & TypeFlags.Union ? every((type as UnionType).types, f) : f(type);
     }
 
+    function someContainedType(type: Type, f: (t: Type) => boolean): boolean {
+        return type.flags & TypeFlags.UnionOrIntersection ? some((type as UnionOrIntersectionType).types, f) : f(type);
+    }
+
     function everyContainedType(type: Type, f: (t: Type) => boolean): boolean {
         return type.flags & TypeFlags.UnionOrIntersection ? every((type as UnionOrIntersectionType).types, f) : f(type);
     }
@@ -47349,14 +47353,14 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
 
     function checkBaseTypeAccessibility(type: Type, node: ExpressionWithTypeArguments) {
         const signatures = getSignaturesOfType(type, SignatureKind.Construct);
-        if (signatures.length) {
-            const declaration = signatures[0].declaration;
-            if (declaration && hasEffectiveModifier(declaration, ModifierFlags.Private)) {
-                const typeClassDeclaration = getClassLikeDeclarationOfSymbol(type.symbol)!;
-                if (!isNodeWithinClass(node, typeClassDeclaration)) {
-                    error(node, Diagnostics.Cannot_extend_a_class_0_Class_constructor_is_marked_as_private, getFullyQualifiedName(type.symbol));
-                }
-            }
+        const isPrivateAccessOutsideOfClass = some(signatures, (s) =>
+            !!s.declaration && hasEffectiveModifier(s.declaration, ModifierFlags.Private) &&
+            someContainedType(type, (t) => {
+                const typeClassDeclaration = getClassLikeDeclarationOfSymbol(t.symbol);
+                return !!typeClassDeclaration && !isNodeWithinClass(node, typeClassDeclaration);
+            }));
+        if (isPrivateAccessOutsideOfClass) {
+            error(node, Diagnostics.Cannot_extend_a_class_0_Class_constructor_is_marked_as_private, type.symbol ? getFullyQualifiedName(type.symbol) : typeToString(type));
         }
     }
 
diff --git a/tests/baselines/reference/extendPrivateConstructorClass2.errors.txt b/tests/baselines/reference/extendPrivateConstructorClass2.errors.txt
new file mode 100644
index 0000000000000..7926be1f167ae
--- /dev/null
+++ b/tests/baselines/reference/extendPrivateConstructorClass2.errors.txt
@@ -0,0 +1,71 @@
+extendPrivateConstructorClass2.ts(10,1): error TS2673: Constructor of class 'A1' is private and only accessible within the class declaration.
+extendPrivateConstructorClass2.ts(11,24): error TS2675: Cannot extend a class 'typeof A1 & typeof B1'. Class constructor is marked as private.
+extendPrivateConstructorClass2.ts(23,24): error TS2675: Cannot extend a class 'typeof A2 & typeof B2'. Class constructor is marked as private.
+extendPrivateConstructorClass2.ts(33,26): error TS2675: Cannot extend a class 'typeof j1 & typeof j2'. Class constructor is marked as private.
+extendPrivateConstructorClass2.ts(44,5): error TS1184: Modifiers cannot appear here.
+extendPrivateConstructorClass2.ts(44,30): error TS2510: Base constructors must all have the same return type.
+extendPrivateConstructorClass2.ts(44,30): error TS2675: Cannot extend a class 'typeof j3 & typeof j4'. Class constructor is marked as private.
+
+
+==== extendPrivateConstructorClass2.ts (7 errors) ====
+    class A1 {
+      private constructor(arg: string) {}
+    }
+    class B1 {
+      constructor(arg: number) {}
+    }
+    
+    declare const Cls1: typeof A1 & typeof B1;
+    
+    new Cls1(42); // error
+    ~~~~~~~~~~~~
+!!! error TS2673: Constructor of class 'A1' is private and only accessible within the class declaration.
+    class Derived1 extends Cls1 {} // error
+                           ~~~~
+!!! error TS2675: Cannot extend a class 'typeof A1 & typeof B1'. Class constructor is marked as private.
+    
+    class A2 {
+      constructor(arg: string) {}
+    }
+    class B2 {
+      private constructor(arg: number) {}
+    }
+    
+    declare const Cls2: typeof A2 & typeof B2;
+    
+    new Cls2(42); // error
+    class Derived2 extends Cls2 {} // error
+                           ~~~~
+!!! error TS2675: Cannot extend a class 'typeof A2 & typeof B2'. Class constructor is marked as private.
+    
+    // https://github.com/microsoft/TypeScript/issues/62614
+    declare abstract class j1 {
+      private constructor(...args: any[]);
+    }
+    declare abstract class j2 {
+      private constructor(...args: any[]);
+    }
+    declare const jS: typeof j1 & typeof j2;
+    declare class j0 extends jS {} // error
+                             ~~
+!!! error TS2675: Cannot extend a class 'typeof j1 & typeof j2'. Class constructor is marked as private.
+    
+    abstract class j3 {
+      private constructor(arg: number) {}
+      method(_any: any) {
+        abstract class j4 {
+          private constructor(arg: number) {}
+        }
+    
+        const jS: typeof j3 & typeof j4 = _any;
+    
+        declare class j0 extends jS {} // bizarre but ok
+        ~~~~~~~
+!!! error TS1184: Modifiers cannot appear here.
+                                 ~~
+!!! error TS2510: Base constructors must all have the same return type.
+                                 ~~
+!!! error TS2675: Cannot extend a class 'typeof j3 & typeof j4'. Class constructor is marked as private.
+      }
+    }
+    
\ No newline at end of file
diff --git a/tests/baselines/reference/extendPrivateConstructorClass2.symbols b/tests/baselines/reference/extendPrivateConstructorClass2.symbols
new file mode 100644
index 0000000000000..2664854503c78
--- /dev/null
+++ b/tests/baselines/reference/extendPrivateConstructorClass2.symbols
@@ -0,0 +1,104 @@
+//// [tests/cases/compiler/extendPrivateConstructorClass2.ts] ////
+
+=== extendPrivateConstructorClass2.ts ===
+class A1 {
+>A1 : Symbol(A1, Decl(extendPrivateConstructorClass2.ts, 0, 0))
+
+  private constructor(arg: string) {}
+>arg : Symbol(arg, Decl(extendPrivateConstructorClass2.ts, 1, 22))
+}
+class B1 {
+>B1 : Symbol(B1, Decl(extendPrivateConstructorClass2.ts, 2, 1))
+
+  constructor(arg: number) {}
+>arg : Symbol(arg, Decl(extendPrivateConstructorClass2.ts, 4, 14))
+}
+
+declare const Cls1: typeof A1 & typeof B1;
+>Cls1 : Symbol(Cls1, Decl(extendPrivateConstructorClass2.ts, 7, 13))
+>A1 : Symbol(A1, Decl(extendPrivateConstructorClass2.ts, 0, 0))
+>B1 : Symbol(B1, Decl(extendPrivateConstructorClass2.ts, 2, 1))
+
+new Cls1(42); // error
+>Cls1 : Symbol(Cls1, Decl(extendPrivateConstructorClass2.ts, 7, 13))
+
+class Derived1 extends Cls1 {} // error
+>Derived1 : Symbol(Derived1, Decl(extendPrivateConstructorClass2.ts, 9, 13))
+>Cls1 : Symbol(Cls1, Decl(extendPrivateConstructorClass2.ts, 7, 13))
+
+class A2 {
+>A2 : Symbol(A2, Decl(extendPrivateConstructorClass2.ts, 10, 30))
+
+  constructor(arg: string) {}
+>arg : Symbol(arg, Decl(extendPrivateConstructorClass2.ts, 13, 14))
+}
+class B2 {
+>B2 : Symbol(B2, Decl(extendPrivateConstructorClass2.ts, 14, 1))
+
+  private constructor(arg: number) {}
+>arg : Symbol(arg, Decl(extendPrivateConstructorClass2.ts, 16, 22))
+}
+
+declare const Cls2: typeof A2 & typeof B2;
+>Cls2 : Symbol(Cls2, Decl(extendPrivateConstructorClass2.ts, 19, 13))
+>A2 : Symbol(A2, Decl(extendPrivateConstructorClass2.ts, 10, 30))
+>B2 : Symbol(B2, Decl(extendPrivateConstructorClass2.ts, 14, 1))
+
+new Cls2(42); // error
+>Cls2 : Symbol(Cls2, Decl(extendPrivateConstructorClass2.ts, 19, 13))
+
+class Derived2 extends Cls2 {} // error
+>Derived2 : Symbol(Derived2, Decl(extendPrivateConstructorClass2.ts, 21, 13))
+>Cls2 : Symbol(Cls2, Decl(extendPrivateConstructorClass2.ts, 19, 13))
+
+// https://github.com/microsoft/TypeScript/issues/62614
+declare abstract class j1 {
+>j1 : Symbol(j1, Decl(extendPrivateConstructorClass2.ts, 22, 30))
+
+  private constructor(...args: any[]);
+>args : Symbol(args, Decl(extendPrivateConstructorClass2.ts, 26, 22))
+}
+declare abstract class j2 {
+>j2 : Symbol(j2, Decl(extendPrivateConstructorClass2.ts, 27, 1))
+
+  private constructor(...args: any[]);
+>args : Symbol(args, Decl(extendPrivateConstructorClass2.ts, 29, 22))
+}
+declare const jS: typeof j1 & typeof j2;
+>jS : Symbol(jS, Decl(extendPrivateConstructorClass2.ts, 31, 13))
+>j1 : Symbol(j1, Decl(extendPrivateConstructorClass2.ts, 22, 30))
+>j2 : Symbol(j2, Decl(extendPrivateConstructorClass2.ts, 27, 1))
+
+declare class j0 extends jS {} // error
+>j0 : Symbol(j0, Decl(extendPrivateConstructorClass2.ts, 31, 40))
+>jS : Symbol(jS, Decl(extendPrivateConstructorClass2.ts, 31, 13))
+
+abstract class j3 {
+>j3 : Symbol(j3, Decl(extendPrivateConstructorClass2.ts, 32, 30))
+
+  private constructor(arg: number) {}
+>arg : Symbol(arg, Decl(extendPrivateConstructorClass2.ts, 35, 22))
+
+  method(_any: any) {
+>method : Symbol(j3.method, Decl(extendPrivateConstructorClass2.ts, 35, 37))
+>_any : Symbol(_any, Decl(extendPrivateConstructorClass2.ts, 36, 9))
+
+    abstract class j4 {
+>j4 : Symbol(j4, Decl(extendPrivateConstructorClass2.ts, 36, 21))
+
+      private constructor(arg: number) {}
+>arg : Symbol(arg, Decl(extendPrivateConstructorClass2.ts, 38, 26))
+    }
+
+    const jS: typeof j3 & typeof j4 = _any;
+>jS : Symbol(jS, Decl(extendPrivateConstructorClass2.ts, 41, 9))
+>j3 : Symbol(j3, Decl(extendPrivateConstructorClass2.ts, 32, 30))
+>j4 : Symbol(j4, Decl(extendPrivateConstructorClass2.ts, 36, 21))
+>_any : Symbol(_any, Decl(extendPrivateConstructorClass2.ts, 36, 9))
+
+    declare class j0 extends jS {} // bizarre but ok
+>j0 : Symbol(j0, Decl(extendPrivateConstructorClass2.ts, 41, 43))
+>jS : Symbol(jS, Decl(extendPrivateConstructorClass2.ts, 41, 9))
+  }
+}
+
diff --git a/tests/baselines/reference/extendPrivateConstructorClass2.types b/tests/baselines/reference/extendPrivateConstructorClass2.types
new file mode 100644
index 0000000000000..0170594e19813
--- /dev/null
+++ b/tests/baselines/reference/extendPrivateConstructorClass2.types
@@ -0,0 +1,153 @@
+//// [tests/cases/compiler/extendPrivateConstructorClass2.ts] ////
+
+=== extendPrivateConstructorClass2.ts ===
+class A1 {
+>A1 : A1
+>   : ^^
+
+  private constructor(arg: string) {}
+>arg : string
+>    : ^^^^^^
+}
+class B1 {
+>B1 : B1
+>   : ^^
+
+  constructor(arg: number) {}
+>arg : number
+>    : ^^^^^^
+}
+
+declare const Cls1: typeof A1 & typeof B1;
+>Cls1 : typeof A1 & typeof B1
+>     : ^^^^^^^^^^^^^^^^^^^^^
+>A1 : typeof A1
+>   : ^^^^^^^^^
+>B1 : typeof B1
+>   : ^^^^^^^^^
+
+new Cls1(42); // error
+>new Cls1(42) : any
+>             : ^^^
+>Cls1 : typeof A1 & typeof B1
+>     : ^^^^^^^^^^^^^^^^^^^^^
+>42 : 42
+>   : ^^
+
+class Derived1 extends Cls1 {} // error
+>Derived1 : Derived1
+>         : ^^^^^^^^
+>Cls1 : A1
+>     : ^^
+
+class A2 {
+>A2 : A2
+>   : ^^
+
+  constructor(arg: string) {}
+>arg : string
+>    : ^^^^^^
+}
+class B2 {
+>B2 : B2
+>   : ^^
+
+  private constructor(arg: number) {}
+>arg : number
+>    : ^^^^^^
+}
+
+declare const Cls2: typeof A2 & typeof B2;
+>Cls2 : typeof A2 & typeof B2
+>     : ^^^^^^^^^^^^^^^^^^^^^
+>A2 : typeof A2
+>   : ^^^^^^^^^
+>B2 : typeof B2
+>   : ^^^^^^^^^
+
+new Cls2(42); // error
+>new Cls2(42) : B2
+>             : ^^
+>Cls2 : typeof A2 & typeof B2
+>     : ^^^^^^^^^^^^^^^^^^^^^
+>42 : 42
+>   : ^^
+
+class Derived2 extends Cls2 {} // error
+>Derived2 : Derived2
+>         : ^^^^^^^^
+>Cls2 : A2
+>     : ^^
+
+// https://github.com/microsoft/TypeScript/issues/62614
+declare abstract class j1 {
+>j1 : j1
+>   : ^^
+
+  private constructor(...args: any[]);
+>args : any[]
+>     : ^^^^^
+}
+declare abstract class j2 {
+>j2 : j2
+>   : ^^
+
+  private constructor(...args: any[]);
+>args : any[]
+>     : ^^^^^
+}
+declare const jS: typeof j1 & typeof j2;
+>jS : typeof j1 & typeof j2
+>   : ^^^^^^^^^^^^^^^^^^^^^
+>j1 : typeof j1
+>   : ^^^^^^^^^
+>j2 : typeof j2
+>   : ^^^^^^^^^
+
+declare class j0 extends jS {} // error
+>j0 : j0
+>   : ^^
+>jS : j1 & j2
+>   : ^^^^^^^
+
+abstract class j3 {
+>j3 : j3
+>   : ^^
+
+  private constructor(arg: number) {}
+>arg : number
+>    : ^^^^^^
+
+  method(_any: any) {
+>method : (_any: any) => void
+>       : ^    ^^   ^^^^^^^^^
+>_any : any
+>     : ^^^
+
+    abstract class j4 {
+>j4 : j4
+>   : ^^
+
+      private constructor(arg: number) {}
+>arg : number
+>    : ^^^^^^
+    }
+
+    const jS: typeof j3 & typeof j4 = _any;
+>jS : typeof j3 & typeof j4
+>   : ^^^^^^^^^^^^^^^^^^^^^
+>j3 : typeof j3
+>   : ^^^^^^^^^
+>j4 : typeof j4
+>   : ^^^^^^^^^
+>_any : any
+>     : ^^^
+
+    declare class j0 extends jS {} // bizarre but ok
+>j0 : j0
+>   : ^^
+>jS : j3
+>   : ^^
+  }
+}
+
diff --git a/tests/cases/compiler/extendPrivateConstructorClass2.ts b/tests/cases/compiler/extendPrivateConstructorClass2.ts
new file mode 100644
index 0000000000000..6057df765331b
--- /dev/null
+++ b/tests/cases/compiler/extendPrivateConstructorClass2.ts
@@ -0,0 +1,49 @@
+// @strict: true
+// @noEmit: true
+
+class A1 {
+  private constructor(arg: string) {}
+}
+class B1 {
+  constructor(arg: number) {}
+}
+
+declare const Cls1: typeof A1 & typeof B1;
+
+new Cls1(42); // error
+class Derived1 extends Cls1 {} // error
+
+class A2 {
+  constructor(arg: string) {}
+}
+class B2 {
+  private constructor(arg: number) {}
+}
+
+declare const Cls2: typeof A2 & typeof B2;
+
+new Cls2(42); // error
+class Derived2 extends Cls2 {} // error
+
+// https://github.com/microsoft/TypeScript/issues/62614
+declare abstract class j1 {
+  private constructor(...args: any[]);
+}
+declare abstract class j2 {
+  private constructor(...args: any[]);
+}
+declare const jS: typeof j1 & typeof j2;
+declare class j0 extends jS {} // error
+
+abstract class j3 {
+  private constructor(arg: number) {}
+  method(_any: any) {
+    abstract class j4 {
+      private constructor(arg: number) {}
+    }
+
+    const jS: typeof j3 & typeof j4 = _any;
+
+    declare class j0 extends jS {} // bizarre but ok
+  }
+}

From 74a85326ded265d4ac5072959f3069cd18dce350 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Mateusz=20Burzy=C5=84ski?= <mateuszburzynski@gmail.com>
Date: Fri, 17 Oct 2025 23:36:08 +0200
Subject: [PATCH 2/6] fix an extra thing

---
 src/compiler/checker.ts                       | 72 ++++++++-----------
 .../classConstructorAccessibility.errors.txt  |  8 +--
 .../extendPrivateConstructorClass2.errors.txt |  9 ++-
 .../extendPrivateConstructorClass2.types      |  4 +-
 .../reference/noCrashOnMixin.errors.txt       |  4 +-
 5 files changed, 45 insertions(+), 52 deletions(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 582cdc04e3108..b2c5978fdfa0d 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -28402,10 +28402,6 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
         return type.flags & TypeFlags.Union ? every((type as UnionType).types, f) : f(type);
     }
 
-    function someContainedType(type: Type, f: (t: Type) => boolean): boolean {
-        return type.flags & TypeFlags.UnionOrIntersection ? some((type as UnionOrIntersectionType).types, f) : f(type);
-    }
-
     function everyContainedType(type: Type, f: (t: Type) => boolean): boolean {
         return type.flags & TypeFlags.UnionOrIntersection ? every((type as UnionOrIntersectionType).types, f) : f(type);
     }
@@ -37060,7 +37056,14 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
         // that the user will not add any.
         const constructSignatures = getSignaturesOfType(expressionType, SignatureKind.Construct);
         if (constructSignatures.length) {
-            if (!isConstructorAccessible(node, constructSignatures[0])) {
+            const accessibilityErrorKind = getConstructorAccessibilityError(node, constructSignatures, ModifierFlags.NonPublicAccessibilityModifier);
+            if (accessibilityErrorKind) {
+                if (accessibilityErrorKind & ModifierFlags.Private) {
+                    error(node, Diagnostics.Constructor_of_class_0_is_private_and_only_accessible_within_the_class_declaration, expressionType.symbol ? getFullyQualifiedName(expressionType.symbol) : typeToString(expressionType));
+                }
+                if (accessibilityErrorKind & ModifierFlags.Protected) {
+                    error(node, Diagnostics.Constructor_of_class_0_is_protected_and_only_accessible_within_the_class_declaration, expressionType.symbol ? getFullyQualifiedName(expressionType.symbol) : typeToString(expressionType));
+                }
                 return resolveErrorCall(node);
             }
             // If the expression is a class of abstract type, or an abstract construct signature,
@@ -37141,41 +37144,34 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
         return typeHasProtectedAccessibleBase(target, firstBase as InterfaceType);
     }
 
-    function isConstructorAccessible(node: NewExpression, signature: Signature) {
-        if (!signature || !signature.declaration) {
-            return true;
-        }
-
-        const declaration = signature.declaration;
-        const modifiers = getSelectedEffectiveModifierFlags(declaration, ModifierFlags.NonPublicAccessibilityModifier);
+    function getConstructorAccessibilityErrorKind(node: Expression, signatures: readonly Signature[], modifiersMask: ModifierFlags) {
+        for (const signature of signatures) {
+            if (!signature.declaration) {
+                continue;
+            }
+            const declaration = signature.declaration;
+            const modifiers = getSelectedEffectiveModifierFlags(declaration, modifiersMask);
 
-        // (1) Public constructors and (2) constructor functions are always accessible.
-        if (!modifiers || declaration.kind !== SyntaxKind.Constructor) {
-            return true;
-        }
+            // (1) Public constructors and (2) constructor functions are always accessible.
+            if (!modifiers || declaration.kind !== SyntaxKind.Constructor) {
+                continue;
+            }
 
-        const declaringClassDeclaration = getClassLikeDeclarationOfSymbol(declaration.parent.symbol)!;
-        const declaringClass = getDeclaredTypeOfSymbol(declaration.parent.symbol) as InterfaceType;
+            const declaringClassDeclaration = getClassLikeDeclarationOfSymbol(declaration.parent.symbol)!;
 
-        // A private or protected constructor can only be instantiated within its own class (or a subclass, for protected)
-        if (!isNodeWithinClass(node, declaringClassDeclaration)) {
-            const containingClass = getContainingClass(node);
-            if (containingClass && modifiers & ModifierFlags.Protected) {
-                const containingType = getTypeOfNode(containingClass);
-                if (typeHasProtectedAccessibleBase(declaration.parent.symbol, containingType as InterfaceType)) {
-                    return true;
+            // A private or protected constructor can only be instantiated within its own class (or a subclass, for protected)
+            if (!isNodeWithinClass(node, declaringClassDeclaration)) {
+                const containingClass = getContainingClass(node);
+                if (containingClass && modifiers & ModifierFlags.Protected) {
+                    const containingType = getTypeOfNode(containingClass);
+                    if (typeHasProtectedAccessibleBase(declaration.parent.symbol, containingType as InterfaceType)) {
+                        continue;
+                    }
                 }
+                return modifiers;
             }
-            if (modifiers & ModifierFlags.Private) {
-                error(node, Diagnostics.Constructor_of_class_0_is_private_and_only_accessible_within_the_class_declaration, typeToString(declaringClass));
-            }
-            if (modifiers & ModifierFlags.Protected) {
-                error(node, Diagnostics.Constructor_of_class_0_is_protected_and_only_accessible_within_the_class_declaration, typeToString(declaringClass));
-            }
-            return false;
         }
-
-        return true;
+        return ModifierFlags.None;
     }
 
     function invocationErrorDetails(errorTarget: Node, apparentType: Type, kind: SignatureKind): { messageChain: DiagnosticMessageChain; relatedMessage: DiagnosticMessage | undefined; } {
@@ -47353,13 +47349,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
 
     function checkBaseTypeAccessibility(type: Type, node: ExpressionWithTypeArguments) {
         const signatures = getSignaturesOfType(type, SignatureKind.Construct);
-        const isPrivateAccessOutsideOfClass = some(signatures, (s) =>
-            !!s.declaration && hasEffectiveModifier(s.declaration, ModifierFlags.Private) &&
-            someContainedType(type, (t) => {
-                const typeClassDeclaration = getClassLikeDeclarationOfSymbol(t.symbol);
-                return !!typeClassDeclaration && !isNodeWithinClass(node, typeClassDeclaration);
-            }));
-        if (isPrivateAccessOutsideOfClass) {
+        if (getConstructorAccessibilityErrorKind(node, signatures, ModifierFlags.Private)) {
             error(node, Diagnostics.Cannot_extend_a_class_0_Class_constructor_is_marked_as_private, type.symbol ? getFullyQualifiedName(type.symbol) : typeToString(type));
         }
     }
diff --git a/tests/baselines/reference/classConstructorAccessibility.errors.txt b/tests/baselines/reference/classConstructorAccessibility.errors.txt
index d3e2ba69434a1..1ad858d234702 100644
--- a/tests/baselines/reference/classConstructorAccessibility.errors.txt
+++ b/tests/baselines/reference/classConstructorAccessibility.errors.txt
@@ -1,7 +1,7 @@
 classConstructorAccessibility.ts(14,9): error TS2673: Constructor of class 'D' is private and only accessible within the class declaration.
 classConstructorAccessibility.ts(15,9): error TS2674: Constructor of class 'E' is protected and only accessible within the class declaration.
-classConstructorAccessibility.ts(31,13): error TS2673: Constructor of class 'D<T>' is private and only accessible within the class declaration.
-classConstructorAccessibility.ts(32,13): error TS2674: Constructor of class 'E<T>' is protected and only accessible within the class declaration.
+classConstructorAccessibility.ts(31,13): error TS2673: Constructor of class 'D' is private and only accessible within the class declaration.
+classConstructorAccessibility.ts(32,13): error TS2674: Constructor of class 'E' is protected and only accessible within the class declaration.
 
 
 ==== classConstructorAccessibility.ts (4 errors) ====
@@ -41,9 +41,9 @@ classConstructorAccessibility.ts(32,13): error TS2674: Constructor of class 'E<T
         var c = new C(1);
         var d = new D(1); // error
                 ~~~~~~~~
-!!! error TS2673: Constructor of class 'D<T>' is private and only accessible within the class declaration.
+!!! error TS2673: Constructor of class 'D' is private and only accessible within the class declaration.
         var e = new E(1); // error
                 ~~~~~~~~
-!!! error TS2674: Constructor of class 'E<T>' is protected and only accessible within the class declaration.
+!!! error TS2674: Constructor of class 'E' is protected and only accessible within the class declaration.
     }
     
\ No newline at end of file
diff --git a/tests/baselines/reference/extendPrivateConstructorClass2.errors.txt b/tests/baselines/reference/extendPrivateConstructorClass2.errors.txt
index 7926be1f167ae..514d4063b355f 100644
--- a/tests/baselines/reference/extendPrivateConstructorClass2.errors.txt
+++ b/tests/baselines/reference/extendPrivateConstructorClass2.errors.txt
@@ -1,5 +1,6 @@
-extendPrivateConstructorClass2.ts(10,1): error TS2673: Constructor of class 'A1' is private and only accessible within the class declaration.
+extendPrivateConstructorClass2.ts(10,1): error TS2673: Constructor of class 'typeof A1 & typeof B1' is private and only accessible within the class declaration.
 extendPrivateConstructorClass2.ts(11,24): error TS2675: Cannot extend a class 'typeof A1 & typeof B1'. Class constructor is marked as private.
+extendPrivateConstructorClass2.ts(22,1): error TS2673: Constructor of class 'typeof A2 & typeof B2' is private and only accessible within the class declaration.
 extendPrivateConstructorClass2.ts(23,24): error TS2675: Cannot extend a class 'typeof A2 & typeof B2'. Class constructor is marked as private.
 extendPrivateConstructorClass2.ts(33,26): error TS2675: Cannot extend a class 'typeof j1 & typeof j2'. Class constructor is marked as private.
 extendPrivateConstructorClass2.ts(44,5): error TS1184: Modifiers cannot appear here.
@@ -7,7 +8,7 @@ extendPrivateConstructorClass2.ts(44,30): error TS2510: Base constructors must a
 extendPrivateConstructorClass2.ts(44,30): error TS2675: Cannot extend a class 'typeof j3 & typeof j4'. Class constructor is marked as private.
 
 
-==== extendPrivateConstructorClass2.ts (7 errors) ====
+==== extendPrivateConstructorClass2.ts (8 errors) ====
     class A1 {
       private constructor(arg: string) {}
     }
@@ -19,7 +20,7 @@ extendPrivateConstructorClass2.ts(44,30): error TS2675: Cannot extend a class 't
     
     new Cls1(42); // error
     ~~~~~~~~~~~~
-!!! error TS2673: Constructor of class 'A1' is private and only accessible within the class declaration.
+!!! error TS2673: Constructor of class 'typeof A1 & typeof B1' is private and only accessible within the class declaration.
     class Derived1 extends Cls1 {} // error
                            ~~~~
 !!! error TS2675: Cannot extend a class 'typeof A1 & typeof B1'. Class constructor is marked as private.
@@ -34,6 +35,8 @@ extendPrivateConstructorClass2.ts(44,30): error TS2675: Cannot extend a class 't
     declare const Cls2: typeof A2 & typeof B2;
     
     new Cls2(42); // error
+    ~~~~~~~~~~~~
+!!! error TS2673: Constructor of class 'typeof A2 & typeof B2' is private and only accessible within the class declaration.
     class Derived2 extends Cls2 {} // error
                            ~~~~
 !!! error TS2675: Cannot extend a class 'typeof A2 & typeof B2'. Class constructor is marked as private.
diff --git a/tests/baselines/reference/extendPrivateConstructorClass2.types b/tests/baselines/reference/extendPrivateConstructorClass2.types
index 0170594e19813..d667b5b0665c2 100644
--- a/tests/baselines/reference/extendPrivateConstructorClass2.types
+++ b/tests/baselines/reference/extendPrivateConstructorClass2.types
@@ -66,8 +66,8 @@ declare const Cls2: typeof A2 & typeof B2;
 >   : ^^^^^^^^^
 
 new Cls2(42); // error
->new Cls2(42) : B2
->             : ^^
+>new Cls2(42) : any
+>             : ^^^
 >Cls2 : typeof A2 & typeof B2
 >     : ^^^^^^^^^^^^^^^^^^^^^
 >42 : 42
diff --git a/tests/baselines/reference/noCrashOnMixin.errors.txt b/tests/baselines/reference/noCrashOnMixin.errors.txt
index cf9bb9cd07499..9023d324c25c8 100644
--- a/tests/baselines/reference/noCrashOnMixin.errors.txt
+++ b/tests/baselines/reference/noCrashOnMixin.errors.txt
@@ -1,4 +1,4 @@
-noCrashOnMixin.ts(21,9): error TS2674: Constructor of class 'Abstract' is protected and only accessible within the class declaration.
+noCrashOnMixin.ts(21,9): error TS2674: Constructor of class 'Concrete' is protected and only accessible within the class declaration.
 
 
 ==== noCrashOnMixin.ts (1 errors) ====
@@ -24,6 +24,6 @@ noCrashOnMixin.ts(21,9): error TS2674: Constructor of class 'Abstract' is protec
         public trigger() {
             new Concrete();
             ~~~~~~~~~~~~~~
-!!! error TS2674: Constructor of class 'Abstract' is protected and only accessible within the class declaration.
+!!! error TS2674: Constructor of class 'Concrete' is protected and only accessible within the class declaration.
         }
     }
\ No newline at end of file

From 80647a81d15bef1eaecd273e6f06eb57fd3b6d5d Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Mateusz=20Burzy=C5=84ski?= <mateuszburzynski@gmail.com>
Date: Fri, 17 Oct 2025 23:36:42 +0200
Subject: [PATCH 3/6] fix reported classes in error messages

---
 src/compiler/checker.ts                       | 26 +++++++++++--------
 .../classConstructorAccessibility.errors.txt  |  8 +++---
 .../extendPrivateConstructorClass2.errors.txt | 24 ++++++++---------
 .../reference/noCrashOnMixin.errors.txt       |  4 +--
 4 files changed, 33 insertions(+), 29 deletions(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index b2c5978fdfa0d..af7df91221824 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -37056,13 +37056,13 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
         // that the user will not add any.
         const constructSignatures = getSignaturesOfType(expressionType, SignatureKind.Construct);
         if (constructSignatures.length) {
-            const accessibilityErrorKind = getConstructorAccessibilityError(node, constructSignatures, ModifierFlags.NonPublicAccessibilityModifier);
-            if (accessibilityErrorKind) {
-                if (accessibilityErrorKind & ModifierFlags.Private) {
-                    error(node, Diagnostics.Constructor_of_class_0_is_private_and_only_accessible_within_the_class_declaration, expressionType.symbol ? getFullyQualifiedName(expressionType.symbol) : typeToString(expressionType));
+            const accessibilityError = getConstructorAccessibilityError(node, constructSignatures, ModifierFlags.NonPublicAccessibilityModifier);
+            if (accessibilityError) {
+                if (accessibilityError.kind & ModifierFlags.Private) {
+                    error(node, Diagnostics.Constructor_of_class_0_is_private_and_only_accessible_within_the_class_declaration, typeToString(accessibilityError.declaringClass));
                 }
-                if (accessibilityErrorKind & ModifierFlags.Protected) {
-                    error(node, Diagnostics.Constructor_of_class_0_is_protected_and_only_accessible_within_the_class_declaration, expressionType.symbol ? getFullyQualifiedName(expressionType.symbol) : typeToString(expressionType));
+                if (accessibilityError.kind & ModifierFlags.Protected) {
+                    error(node, Diagnostics.Constructor_of_class_0_is_protected_and_only_accessible_within_the_class_declaration, typeToString(accessibilityError.declaringClass));
                 }
                 return resolveErrorCall(node);
             }
@@ -37144,7 +37144,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
         return typeHasProtectedAccessibleBase(target, firstBase as InterfaceType);
     }
 
-    function getConstructorAccessibilityErrorKind(node: Expression, signatures: readonly Signature[], modifiersMask: ModifierFlags) {
+    function getConstructorAccessibilityError(node: Expression, signatures: readonly Signature[], modifiersMask: ModifierFlags) {
         for (const signature of signatures) {
             if (!signature.declaration) {
                 continue;
@@ -37168,10 +37168,13 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
                         continue;
                     }
                 }
-                return modifiers;
+                return {
+                    kind: modifiers,
+                    declaringClass: getDeclaredTypeOfSymbol(declaration.parent.symbol),
+                }
             }
         }
-        return ModifierFlags.None;
+        return undefined;
     }
 
     function invocationErrorDetails(errorTarget: Node, apparentType: Type, kind: SignatureKind): { messageChain: DiagnosticMessageChain; relatedMessage: DiagnosticMessage | undefined; } {
@@ -47349,8 +47352,9 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
 
     function checkBaseTypeAccessibility(type: Type, node: ExpressionWithTypeArguments) {
         const signatures = getSignaturesOfType(type, SignatureKind.Construct);
-        if (getConstructorAccessibilityErrorKind(node, signatures, ModifierFlags.Private)) {
-            error(node, Diagnostics.Cannot_extend_a_class_0_Class_constructor_is_marked_as_private, type.symbol ? getFullyQualifiedName(type.symbol) : typeToString(type));
+        const accessibilityError = getConstructorAccessibilityError(node, signatures, ModifierFlags.Private);
+        if (accessibilityError) {
+            error(node, Diagnostics.Cannot_extend_a_class_0_Class_constructor_is_marked_as_private, getFullyQualifiedName(accessibilityError.declaringClass.symbol));
         }
     }
 
diff --git a/tests/baselines/reference/classConstructorAccessibility.errors.txt b/tests/baselines/reference/classConstructorAccessibility.errors.txt
index 1ad858d234702..d3e2ba69434a1 100644
--- a/tests/baselines/reference/classConstructorAccessibility.errors.txt
+++ b/tests/baselines/reference/classConstructorAccessibility.errors.txt
@@ -1,7 +1,7 @@
 classConstructorAccessibility.ts(14,9): error TS2673: Constructor of class 'D' is private and only accessible within the class declaration.
 classConstructorAccessibility.ts(15,9): error TS2674: Constructor of class 'E' is protected and only accessible within the class declaration.
-classConstructorAccessibility.ts(31,13): error TS2673: Constructor of class 'D' is private and only accessible within the class declaration.
-classConstructorAccessibility.ts(32,13): error TS2674: Constructor of class 'E' is protected and only accessible within the class declaration.
+classConstructorAccessibility.ts(31,13): error TS2673: Constructor of class 'D<T>' is private and only accessible within the class declaration.
+classConstructorAccessibility.ts(32,13): error TS2674: Constructor of class 'E<T>' is protected and only accessible within the class declaration.
 
 
 ==== classConstructorAccessibility.ts (4 errors) ====
@@ -41,9 +41,9 @@ classConstructorAccessibility.ts(32,13): error TS2674: Constructor of class 'E'
         var c = new C(1);
         var d = new D(1); // error
                 ~~~~~~~~
-!!! error TS2673: Constructor of class 'D' is private and only accessible within the class declaration.
+!!! error TS2673: Constructor of class 'D<T>' is private and only accessible within the class declaration.
         var e = new E(1); // error
                 ~~~~~~~~
-!!! error TS2674: Constructor of class 'E' is protected and only accessible within the class declaration.
+!!! error TS2674: Constructor of class 'E<T>' is protected and only accessible within the class declaration.
     }
     
\ No newline at end of file
diff --git a/tests/baselines/reference/extendPrivateConstructorClass2.errors.txt b/tests/baselines/reference/extendPrivateConstructorClass2.errors.txt
index 514d4063b355f..1a8c4b60f98be 100644
--- a/tests/baselines/reference/extendPrivateConstructorClass2.errors.txt
+++ b/tests/baselines/reference/extendPrivateConstructorClass2.errors.txt
@@ -1,11 +1,11 @@
-extendPrivateConstructorClass2.ts(10,1): error TS2673: Constructor of class 'typeof A1 & typeof B1' is private and only accessible within the class declaration.
-extendPrivateConstructorClass2.ts(11,24): error TS2675: Cannot extend a class 'typeof A1 & typeof B1'. Class constructor is marked as private.
-extendPrivateConstructorClass2.ts(22,1): error TS2673: Constructor of class 'typeof A2 & typeof B2' is private and only accessible within the class declaration.
-extendPrivateConstructorClass2.ts(23,24): error TS2675: Cannot extend a class 'typeof A2 & typeof B2'. Class constructor is marked as private.
-extendPrivateConstructorClass2.ts(33,26): error TS2675: Cannot extend a class 'typeof j1 & typeof j2'. Class constructor is marked as private.
+extendPrivateConstructorClass2.ts(10,1): error TS2673: Constructor of class 'A1' is private and only accessible within the class declaration.
+extendPrivateConstructorClass2.ts(11,24): error TS2675: Cannot extend a class 'A1'. Class constructor is marked as private.
+extendPrivateConstructorClass2.ts(22,1): error TS2673: Constructor of class 'B2' is private and only accessible within the class declaration.
+extendPrivateConstructorClass2.ts(23,24): error TS2675: Cannot extend a class 'B2'. Class constructor is marked as private.
+extendPrivateConstructorClass2.ts(33,26): error TS2675: Cannot extend a class 'j1'. Class constructor is marked as private.
 extendPrivateConstructorClass2.ts(44,5): error TS1184: Modifiers cannot appear here.
 extendPrivateConstructorClass2.ts(44,30): error TS2510: Base constructors must all have the same return type.
-extendPrivateConstructorClass2.ts(44,30): error TS2675: Cannot extend a class 'typeof j3 & typeof j4'. Class constructor is marked as private.
+extendPrivateConstructorClass2.ts(44,30): error TS2675: Cannot extend a class 'j4'. Class constructor is marked as private.
 
 
 ==== extendPrivateConstructorClass2.ts (8 errors) ====
@@ -20,10 +20,10 @@ extendPrivateConstructorClass2.ts(44,30): error TS2675: Cannot extend a class 't
     
     new Cls1(42); // error
     ~~~~~~~~~~~~
-!!! error TS2673: Constructor of class 'typeof A1 & typeof B1' is private and only accessible within the class declaration.
+!!! error TS2673: Constructor of class 'A1' is private and only accessible within the class declaration.
     class Derived1 extends Cls1 {} // error
                            ~~~~
-!!! error TS2675: Cannot extend a class 'typeof A1 & typeof B1'. Class constructor is marked as private.
+!!! error TS2675: Cannot extend a class 'A1'. Class constructor is marked as private.
     
     class A2 {
       constructor(arg: string) {}
@@ -36,10 +36,10 @@ extendPrivateConstructorClass2.ts(44,30): error TS2675: Cannot extend a class 't
     
     new Cls2(42); // error
     ~~~~~~~~~~~~
-!!! error TS2673: Constructor of class 'typeof A2 & typeof B2' is private and only accessible within the class declaration.
+!!! error TS2673: Constructor of class 'B2' is private and only accessible within the class declaration.
     class Derived2 extends Cls2 {} // error
                            ~~~~
-!!! error TS2675: Cannot extend a class 'typeof A2 & typeof B2'. Class constructor is marked as private.
+!!! error TS2675: Cannot extend a class 'B2'. Class constructor is marked as private.
     
     // https://github.com/microsoft/TypeScript/issues/62614
     declare abstract class j1 {
@@ -51,7 +51,7 @@ extendPrivateConstructorClass2.ts(44,30): error TS2675: Cannot extend a class 't
     declare const jS: typeof j1 & typeof j2;
     declare class j0 extends jS {} // error
                              ~~
-!!! error TS2675: Cannot extend a class 'typeof j1 & typeof j2'. Class constructor is marked as private.
+!!! error TS2675: Cannot extend a class 'j1'. Class constructor is marked as private.
     
     abstract class j3 {
       private constructor(arg: number) {}
@@ -68,7 +68,7 @@ extendPrivateConstructorClass2.ts(44,30): error TS2675: Cannot extend a class 't
                                  ~~
 !!! error TS2510: Base constructors must all have the same return type.
                                  ~~
-!!! error TS2675: Cannot extend a class 'typeof j3 & typeof j4'. Class constructor is marked as private.
+!!! error TS2675: Cannot extend a class 'j4'. Class constructor is marked as private.
       }
     }
     
\ No newline at end of file
diff --git a/tests/baselines/reference/noCrashOnMixin.errors.txt b/tests/baselines/reference/noCrashOnMixin.errors.txt
index 9023d324c25c8..cf9bb9cd07499 100644
--- a/tests/baselines/reference/noCrashOnMixin.errors.txt
+++ b/tests/baselines/reference/noCrashOnMixin.errors.txt
@@ -1,4 +1,4 @@
-noCrashOnMixin.ts(21,9): error TS2674: Constructor of class 'Concrete' is protected and only accessible within the class declaration.
+noCrashOnMixin.ts(21,9): error TS2674: Constructor of class 'Abstract' is protected and only accessible within the class declaration.
 
 
 ==== noCrashOnMixin.ts (1 errors) ====
@@ -24,6 +24,6 @@ noCrashOnMixin.ts(21,9): error TS2674: Constructor of class 'Concrete' is protec
         public trigger() {
             new Concrete();
             ~~~~~~~~~~~~~~
-!!! error TS2674: Constructor of class 'Concrete' is protected and only accessible within the class declaration.
+!!! error TS2674: Constructor of class 'Abstract' is protected and only accessible within the class declaration.
         }
     }
\ No newline at end of file

From a3add66226192ac0b38396891163c71f75a72966 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Mateusz=20Burzy=C5=84ski?= <mateuszburzynski@gmail.com>
Date: Fri, 17 Oct 2025 23:48:30 +0200
Subject: [PATCH 4/6] tweak tests

---
 .../extendPrivateConstructorClass2.errors.txt | 43 +++++---
 .../extendPrivateConstructorClass2.symbols    | 83 +++++++++++++---
 .../extendPrivateConstructorClass2.types      | 99 ++++++++++++++++---
 .../extendPrivateConstructorClass2.ts         | 32 +++++-
 4 files changed, 206 insertions(+), 51 deletions(-)

diff --git a/tests/baselines/reference/extendPrivateConstructorClass2.errors.txt b/tests/baselines/reference/extendPrivateConstructorClass2.errors.txt
index 1a8c4b60f98be..e630f98a03273 100644
--- a/tests/baselines/reference/extendPrivateConstructorClass2.errors.txt
+++ b/tests/baselines/reference/extendPrivateConstructorClass2.errors.txt
@@ -3,12 +3,9 @@ extendPrivateConstructorClass2.ts(11,24): error TS2675: Cannot extend a class 'A
 extendPrivateConstructorClass2.ts(22,1): error TS2673: Constructor of class 'B2' is private and only accessible within the class declaration.
 extendPrivateConstructorClass2.ts(23,24): error TS2675: Cannot extend a class 'B2'. Class constructor is marked as private.
 extendPrivateConstructorClass2.ts(33,26): error TS2675: Cannot extend a class 'j1'. Class constructor is marked as private.
-extendPrivateConstructorClass2.ts(44,5): error TS1184: Modifiers cannot appear here.
-extendPrivateConstructorClass2.ts(44,30): error TS2510: Base constructors must all have the same return type.
-extendPrivateConstructorClass2.ts(44,30): error TS2675: Cannot extend a class 'j4'. Class constructor is marked as private.
 
 
-==== extendPrivateConstructorClass2.ts (8 errors) ====
+==== extendPrivateConstructorClass2.ts (5 errors) ====
     class A1 {
       private constructor(arg: string) {}
     }
@@ -54,21 +51,37 @@ extendPrivateConstructorClass2.ts(44,30): error TS2675: Cannot extend a class 'j
 !!! error TS2675: Cannot extend a class 'j1'. Class constructor is marked as private.
     
     abstract class j3 {
-      private constructor(arg: number) {}
-      method(_any: any) {
+      private constructor(...args: any[]) {}
+      method1() {
         abstract class j4 {
-          private constructor(arg: number) {}
+          private constructor(...args: any[]) {}
+          method2(_any: any) {
+            const jS: typeof j3 & typeof j4 = _any;
+    
+            // bizarre but ok
+            class j0 extends jS {
+              method1() {}
+              method2() {}
+            }
+          }
         }
+      }
+    }
     
-        const jS: typeof j3 & typeof j4 = _any;
+    abstract class j5 {
+      private constructor(...args: any[]) {}
+      method1(_any: any) {
+        abstract class j6 {
+          private constructor(...args: any[]) {}
+          method2() {}
+        }
+        const jS: typeof j5 & typeof j6 = _any;
     
-        declare class j0 extends jS {} // bizarre but ok
-        ~~~~~~~
-!!! error TS1184: Modifiers cannot appear here.
-                                 ~~
-!!! error TS2510: Base constructors must all have the same return type.
-                                 ~~
-!!! error TS2675: Cannot extend a class 'j4'. Class constructor is marked as private.
+        // bizarre but ok too given the base is a result of a mixin
+        class j0 extends jS {
+          method1() {}
+          method2() {}
+        }
       }
     }
     
\ No newline at end of file
diff --git a/tests/baselines/reference/extendPrivateConstructorClass2.symbols b/tests/baselines/reference/extendPrivateConstructorClass2.symbols
index 2664854503c78..e33455fafbb50 100644
--- a/tests/baselines/reference/extendPrivateConstructorClass2.symbols
+++ b/tests/baselines/reference/extendPrivateConstructorClass2.symbols
@@ -76,29 +76,80 @@ declare class j0 extends jS {} // error
 abstract class j3 {
 >j3 : Symbol(j3, Decl(extendPrivateConstructorClass2.ts, 32, 30))
 
-  private constructor(arg: number) {}
->arg : Symbol(arg, Decl(extendPrivateConstructorClass2.ts, 35, 22))
+  private constructor(...args: any[]) {}
+>args : Symbol(args, Decl(extendPrivateConstructorClass2.ts, 35, 22))
 
-  method(_any: any) {
->method : Symbol(j3.method, Decl(extendPrivateConstructorClass2.ts, 35, 37))
->_any : Symbol(_any, Decl(extendPrivateConstructorClass2.ts, 36, 9))
+  method1() {
+>method1 : Symbol(j3.method1, Decl(extendPrivateConstructorClass2.ts, 35, 40))
 
     abstract class j4 {
->j4 : Symbol(j4, Decl(extendPrivateConstructorClass2.ts, 36, 21))
+>j4 : Symbol(j4, Decl(extendPrivateConstructorClass2.ts, 36, 13))
 
-      private constructor(arg: number) {}
->arg : Symbol(arg, Decl(extendPrivateConstructorClass2.ts, 38, 26))
-    }
+      private constructor(...args: any[]) {}
+>args : Symbol(args, Decl(extendPrivateConstructorClass2.ts, 38, 26))
 
-    const jS: typeof j3 & typeof j4 = _any;
->jS : Symbol(jS, Decl(extendPrivateConstructorClass2.ts, 41, 9))
+      method2(_any: any) {
+>method2 : Symbol(j4.method2, Decl(extendPrivateConstructorClass2.ts, 38, 44))
+>_any : Symbol(_any, Decl(extendPrivateConstructorClass2.ts, 39, 14))
+
+        const jS: typeof j3 & typeof j4 = _any;
+>jS : Symbol(jS, Decl(extendPrivateConstructorClass2.ts, 40, 13))
 >j3 : Symbol(j3, Decl(extendPrivateConstructorClass2.ts, 32, 30))
->j4 : Symbol(j4, Decl(extendPrivateConstructorClass2.ts, 36, 21))
->_any : Symbol(_any, Decl(extendPrivateConstructorClass2.ts, 36, 9))
+>j4 : Symbol(j4, Decl(extendPrivateConstructorClass2.ts, 36, 13))
+>_any : Symbol(_any, Decl(extendPrivateConstructorClass2.ts, 39, 14))
+
+        // bizarre but ok
+        class j0 extends jS {
+>j0 : Symbol(j0, Decl(extendPrivateConstructorClass2.ts, 40, 47))
+>jS : Symbol(jS, Decl(extendPrivateConstructorClass2.ts, 40, 13))
+
+          method1() {}
+>method1 : Symbol(j0.method1, Decl(extendPrivateConstructorClass2.ts, 43, 29))
+
+          method2() {}
+>method2 : Symbol(j0.method2, Decl(extendPrivateConstructorClass2.ts, 44, 22))
+        }
+      }
+    }
+  }
+}
+
+abstract class j5 {
+>j5 : Symbol(j5, Decl(extendPrivateConstructorClass2.ts, 50, 1))
+
+  private constructor(...args: any[]) {}
+>args : Symbol(args, Decl(extendPrivateConstructorClass2.ts, 53, 22))
+
+  method1(_any: any) {
+>method1 : Symbol(j5.method1, Decl(extendPrivateConstructorClass2.ts, 53, 40))
+>_any : Symbol(_any, Decl(extendPrivateConstructorClass2.ts, 54, 10))
 
-    declare class j0 extends jS {} // bizarre but ok
->j0 : Symbol(j0, Decl(extendPrivateConstructorClass2.ts, 41, 43))
->jS : Symbol(jS, Decl(extendPrivateConstructorClass2.ts, 41, 9))
+    abstract class j6 {
+>j6 : Symbol(j6, Decl(extendPrivateConstructorClass2.ts, 54, 22))
+
+      private constructor(...args: any[]) {}
+>args : Symbol(args, Decl(extendPrivateConstructorClass2.ts, 56, 26))
+
+      method2() {}
+>method2 : Symbol(j6.method2, Decl(extendPrivateConstructorClass2.ts, 56, 44))
+    }
+    const jS: typeof j5 & typeof j6 = _any;
+>jS : Symbol(jS, Decl(extendPrivateConstructorClass2.ts, 59, 9))
+>j5 : Symbol(j5, Decl(extendPrivateConstructorClass2.ts, 50, 1))
+>j6 : Symbol(j6, Decl(extendPrivateConstructorClass2.ts, 54, 22))
+>_any : Symbol(_any, Decl(extendPrivateConstructorClass2.ts, 54, 10))
+
+    // bizarre but ok too given the base is a result of a mixin
+    class j0 extends jS {
+>j0 : Symbol(j0, Decl(extendPrivateConstructorClass2.ts, 59, 43))
+>jS : Symbol(jS, Decl(extendPrivateConstructorClass2.ts, 59, 9))
+
+      method1() {}
+>method1 : Symbol(j0.method1, Decl(extendPrivateConstructorClass2.ts, 62, 25))
+
+      method2() {}
+>method2 : Symbol(j0.method2, Decl(extendPrivateConstructorClass2.ts, 63, 18))
+    }
   }
 }
 
diff --git a/tests/baselines/reference/extendPrivateConstructorClass2.types b/tests/baselines/reference/extendPrivateConstructorClass2.types
index d667b5b0665c2..288803e0b0f75 100644
--- a/tests/baselines/reference/extendPrivateConstructorClass2.types
+++ b/tests/baselines/reference/extendPrivateConstructorClass2.types
@@ -114,26 +114,29 @@ abstract class j3 {
 >j3 : j3
 >   : ^^
 
-  private constructor(arg: number) {}
->arg : number
->    : ^^^^^^
+  private constructor(...args: any[]) {}
+>args : any[]
+>     : ^^^^^
 
-  method(_any: any) {
->method : (_any: any) => void
->       : ^    ^^   ^^^^^^^^^
->_any : any
->     : ^^^
+  method1() {
+>method1 : () => void
+>        : ^^^^^^^^^^
 
     abstract class j4 {
 >j4 : j4
 >   : ^^
 
-      private constructor(arg: number) {}
->arg : number
->    : ^^^^^^
-    }
+      private constructor(...args: any[]) {}
+>args : any[]
+>     : ^^^^^
 
-    const jS: typeof j3 & typeof j4 = _any;
+      method2(_any: any) {
+>method2 : (_any: any) => void
+>        : ^    ^^   ^^^^^^^^^
+>_any : any
+>     : ^^^
+
+        const jS: typeof j3 & typeof j4 = _any;
 >jS : typeof j3 & typeof j4
 >   : ^^^^^^^^^^^^^^^^^^^^^
 >j3 : typeof j3
@@ -143,11 +146,77 @@ abstract class j3 {
 >_any : any
 >     : ^^^
 
-    declare class j0 extends jS {} // bizarre but ok
+        // bizarre but ok
+        class j0 extends jS {
 >j0 : j0
 >   : ^^
->jS : j3
+>jS : j3 & j4
+>   : ^^^^^^^
+
+          method1() {}
+>method1 : () => void
+>        : ^^^^^^^^^^
+
+          method2() {}
+>method2 : () => void
+>        : ^^^^^^^^^^
+        }
+      }
+    }
+  }
+}
+
+abstract class j5 {
+>j5 : j5
 >   : ^^
+
+  private constructor(...args: any[]) {}
+>args : any[]
+>     : ^^^^^
+
+  method1(_any: any) {
+>method1 : (_any: any) => void
+>        : ^    ^^   ^^^^^^^^^
+>_any : any
+>     : ^^^
+
+    abstract class j6 {
+>j6 : j6
+>   : ^^
+
+      private constructor(...args: any[]) {}
+>args : any[]
+>     : ^^^^^
+
+      method2() {}
+>method2 : () => void
+>        : ^^^^^^^^^^
+    }
+    const jS: typeof j5 & typeof j6 = _any;
+>jS : typeof j5 & typeof j6
+>   : ^^^^^^^^^^^^^^^^^^^^^
+>j5 : typeof j5
+>   : ^^^^^^^^^
+>j6 : typeof j6
+>   : ^^^^^^^^^
+>_any : any
+>     : ^^^
+
+    // bizarre but ok too given the base is a result of a mixin
+    class j0 extends jS {
+>j0 : j0
+>   : ^^
+>jS : j5 & j6
+>   : ^^^^^^^
+
+      method1() {}
+>method1 : () => void
+>        : ^^^^^^^^^^
+
+      method2() {}
+>method2 : () => void
+>        : ^^^^^^^^^^
+    }
   }
 }
 
diff --git a/tests/cases/compiler/extendPrivateConstructorClass2.ts b/tests/cases/compiler/extendPrivateConstructorClass2.ts
index 6057df765331b..9e060cbdb29f3 100644
--- a/tests/cases/compiler/extendPrivateConstructorClass2.ts
+++ b/tests/cases/compiler/extendPrivateConstructorClass2.ts
@@ -36,14 +36,36 @@ declare const jS: typeof j1 & typeof j2;
 declare class j0 extends jS {} // error
 
 abstract class j3 {
-  private constructor(arg: number) {}
-  method(_any: any) {
+  private constructor(...args: any[]) {}
+  method1() {
     abstract class j4 {
-      private constructor(arg: number) {}
+      private constructor(...args: any[]) {}
+      method2(_any: any) {
+        const jS: typeof j3 & typeof j4 = _any;
+
+        // bizarre but ok
+        class j0 extends jS {
+          method1() {}
+          method2() {}
+        }
+      }
     }
+  }
+}
 
-    const jS: typeof j3 & typeof j4 = _any;
+abstract class j5 {
+  private constructor(...args: any[]) {}
+  method1(_any: any) {
+    abstract class j6 {
+      private constructor(...args: any[]) {}
+      method2() {}
+    }
+    const jS: typeof j5 & typeof j6 = _any;
 
-    declare class j0 extends jS {} // bizarre but ok
+    // bizarre but ok too given the base is a result of a mixin
+    class j0 extends jS {
+      method1() {}
+      method2() {}
+    }
   }
 }

From 5a18e53939249e429038c7458e5a4f6d5d16b2bf Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Mateusz=20Burzy=C5=84ski?= <mateuszburzynski@gmail.com>
Date: Fri, 17 Oct 2025 23:58:19 +0200
Subject: [PATCH 5/6] extra tests

---
 .../extendPrivateConstructorClass2.errors.txt |  85 ++++++-
 .../extendPrivateConstructorClass2.symbols    | 149 ++++++++++++-
 .../extendPrivateConstructorClass2.types      | 209 ++++++++++++++++--
 .../extendPrivateConstructorClass2.ts         |  71 +++++-
 4 files changed, 477 insertions(+), 37 deletions(-)

diff --git a/tests/baselines/reference/extendPrivateConstructorClass2.errors.txt b/tests/baselines/reference/extendPrivateConstructorClass2.errors.txt
index e630f98a03273..b8da61049b8c6 100644
--- a/tests/baselines/reference/extendPrivateConstructorClass2.errors.txt
+++ b/tests/baselines/reference/extendPrivateConstructorClass2.errors.txt
@@ -3,9 +3,13 @@ extendPrivateConstructorClass2.ts(11,24): error TS2675: Cannot extend a class 'A
 extendPrivateConstructorClass2.ts(22,1): error TS2673: Constructor of class 'B2' is private and only accessible within the class declaration.
 extendPrivateConstructorClass2.ts(23,24): error TS2675: Cannot extend a class 'B2'. Class constructor is marked as private.
 extendPrivateConstructorClass2.ts(33,26): error TS2675: Cannot extend a class 'j1'. Class constructor is marked as private.
+extendPrivateConstructorClass2.ts(79,26): error TS2510: Base constructors must all have the same return type.
+extendPrivateConstructorClass2.ts(98,22): error TS2510: Base constructors must all have the same return type.
+extendPrivateConstructorClass2.ts(98,22): error TS2675: Cannot extend a class 'j10'. Class constructor is marked as private.
+extendPrivateConstructorClass2.ts(129,22): error TS2675: Cannot extend a class 'j14'. Class constructor is marked as private.
 
 
-==== extendPrivateConstructorClass2.ts (5 errors) ====
+==== extendPrivateConstructorClass2.ts (9 errors) ====
     class A1 {
       private constructor(arg: string) {}
     }
@@ -55,8 +59,8 @@ extendPrivateConstructorClass2.ts(33,26): error TS2675: Cannot extend a class 'j
       method1() {
         abstract class j4 {
           private constructor(...args: any[]) {}
-          method2(_any: any) {
-            const jS: typeof j3 & typeof j4 = _any;
+          method2() {
+            const jS: typeof j3 & typeof j4 = null!;
     
             // bizarre but ok
             class j0 extends jS {
@@ -70,12 +74,12 @@ extendPrivateConstructorClass2.ts(33,26): error TS2675: Cannot extend a class 'j
     
     abstract class j5 {
       private constructor(...args: any[]) {}
-      method1(_any: any) {
+      method1() {
         abstract class j6 {
           private constructor(...args: any[]) {}
           method2() {}
         }
-        const jS: typeof j5 & typeof j6 = _any;
+        const jS: typeof j5 & typeof j6 = null!;
     
         // bizarre but ok too given the base is a result of a mixin
         class j0 extends jS {
@@ -84,4 +88,75 @@ extendPrivateConstructorClass2.ts(33,26): error TS2675: Cannot extend a class 'j
         }
       }
     }
+    
+    abstract class j7 {
+      private constructor(arg: string) {}
+      method1() {
+        abstract class j8 {
+          private constructor(arg: number) {}
+          method2() {
+            const jS: typeof j7 & typeof j8 = null!;
+    
+            // error
+            class j0 extends jS {
+                             ~~
+!!! error TS2510: Base constructors must all have the same return type.
+              method1() {}
+              method2() {}
+            }
+          }
+        }
+      }
+    }
+    
+    abstract class j9 {
+      private constructor(arg: string) {}
+      method1() {
+        abstract class j10 {
+          private constructor(arg: number) {}
+          method2() {}
+        }
+        const jS: typeof j9 & typeof j10 = null!;
+    
+        // error
+        class j0 extends jS {
+                         ~~
+!!! error TS2510: Base constructors must all have the same return type.
+                         ~~
+!!! error TS2675: Cannot extend a class 'j10'. Class constructor is marked as private.
+          method1() {}
+          method2() {}
+        }
+      }
+    }
+    
+    abstract class j11 {
+      private constructor(arg: string) {}
+      static {
+        abstract class j12 {
+          private constructor(arg: number) {}
+          static {
+            const jS: typeof j11 & typeof j12 = null!;
+    
+            // ok
+            class j0 extends jS {}
+          }
+        }
+      }
+    }
+    
+    abstract class j13 {
+      private constructor(arg: string) {}
+      static {
+        abstract class j14 {
+          private constructor(arg: number) {}
+        }
+        const jS: typeof j13 & typeof j14 = null!;
+    
+        // error
+        class j0 extends jS {}
+                         ~~
+!!! error TS2675: Cannot extend a class 'j14'. Class constructor is marked as private.
+      }
+    }
     
\ No newline at end of file
diff --git a/tests/baselines/reference/extendPrivateConstructorClass2.symbols b/tests/baselines/reference/extendPrivateConstructorClass2.symbols
index e33455fafbb50..08ec1ab380651 100644
--- a/tests/baselines/reference/extendPrivateConstructorClass2.symbols
+++ b/tests/baselines/reference/extendPrivateConstructorClass2.symbols
@@ -88,19 +88,17 @@ abstract class j3 {
       private constructor(...args: any[]) {}
 >args : Symbol(args, Decl(extendPrivateConstructorClass2.ts, 38, 26))
 
-      method2(_any: any) {
+      method2() {
 >method2 : Symbol(j4.method2, Decl(extendPrivateConstructorClass2.ts, 38, 44))
->_any : Symbol(_any, Decl(extendPrivateConstructorClass2.ts, 39, 14))
 
-        const jS: typeof j3 & typeof j4 = _any;
+        const jS: typeof j3 & typeof j4 = null!;
 >jS : Symbol(jS, Decl(extendPrivateConstructorClass2.ts, 40, 13))
 >j3 : Symbol(j3, Decl(extendPrivateConstructorClass2.ts, 32, 30))
 >j4 : Symbol(j4, Decl(extendPrivateConstructorClass2.ts, 36, 13))
->_any : Symbol(_any, Decl(extendPrivateConstructorClass2.ts, 39, 14))
 
         // bizarre but ok
         class j0 extends jS {
->j0 : Symbol(j0, Decl(extendPrivateConstructorClass2.ts, 40, 47))
+>j0 : Symbol(j0, Decl(extendPrivateConstructorClass2.ts, 40, 48))
 >jS : Symbol(jS, Decl(extendPrivateConstructorClass2.ts, 40, 13))
 
           method1() {}
@@ -120,12 +118,11 @@ abstract class j5 {
   private constructor(...args: any[]) {}
 >args : Symbol(args, Decl(extendPrivateConstructorClass2.ts, 53, 22))
 
-  method1(_any: any) {
+  method1() {
 >method1 : Symbol(j5.method1, Decl(extendPrivateConstructorClass2.ts, 53, 40))
->_any : Symbol(_any, Decl(extendPrivateConstructorClass2.ts, 54, 10))
 
     abstract class j6 {
->j6 : Symbol(j6, Decl(extendPrivateConstructorClass2.ts, 54, 22))
+>j6 : Symbol(j6, Decl(extendPrivateConstructorClass2.ts, 54, 13))
 
       private constructor(...args: any[]) {}
 >args : Symbol(args, Decl(extendPrivateConstructorClass2.ts, 56, 26))
@@ -133,15 +130,14 @@ abstract class j5 {
       method2() {}
 >method2 : Symbol(j6.method2, Decl(extendPrivateConstructorClass2.ts, 56, 44))
     }
-    const jS: typeof j5 & typeof j6 = _any;
+    const jS: typeof j5 & typeof j6 = null!;
 >jS : Symbol(jS, Decl(extendPrivateConstructorClass2.ts, 59, 9))
 >j5 : Symbol(j5, Decl(extendPrivateConstructorClass2.ts, 50, 1))
->j6 : Symbol(j6, Decl(extendPrivateConstructorClass2.ts, 54, 22))
->_any : Symbol(_any, Decl(extendPrivateConstructorClass2.ts, 54, 10))
+>j6 : Symbol(j6, Decl(extendPrivateConstructorClass2.ts, 54, 13))
 
     // bizarre but ok too given the base is a result of a mixin
     class j0 extends jS {
->j0 : Symbol(j0, Decl(extendPrivateConstructorClass2.ts, 59, 43))
+>j0 : Symbol(j0, Decl(extendPrivateConstructorClass2.ts, 59, 44))
 >jS : Symbol(jS, Decl(extendPrivateConstructorClass2.ts, 59, 9))
 
       method1() {}
@@ -153,3 +149,132 @@ abstract class j5 {
   }
 }
 
+abstract class j7 {
+>j7 : Symbol(j7, Decl(extendPrivateConstructorClass2.ts, 67, 1))
+
+  private constructor(arg: string) {}
+>arg : Symbol(arg, Decl(extendPrivateConstructorClass2.ts, 70, 22))
+
+  method1() {
+>method1 : Symbol(j7.method1, Decl(extendPrivateConstructorClass2.ts, 70, 37))
+
+    abstract class j8 {
+>j8 : Symbol(j8, Decl(extendPrivateConstructorClass2.ts, 71, 13))
+
+      private constructor(arg: number) {}
+>arg : Symbol(arg, Decl(extendPrivateConstructorClass2.ts, 73, 26))
+
+      method2() {
+>method2 : Symbol(j8.method2, Decl(extendPrivateConstructorClass2.ts, 73, 41))
+
+        const jS: typeof j7 & typeof j8 = null!;
+>jS : Symbol(jS, Decl(extendPrivateConstructorClass2.ts, 75, 13))
+>j7 : Symbol(j7, Decl(extendPrivateConstructorClass2.ts, 67, 1))
+>j8 : Symbol(j8, Decl(extendPrivateConstructorClass2.ts, 71, 13))
+
+        // error
+        class j0 extends jS {
+>j0 : Symbol(j0, Decl(extendPrivateConstructorClass2.ts, 75, 48))
+>jS : Symbol(jS, Decl(extendPrivateConstructorClass2.ts, 75, 13))
+
+          method1() {}
+>method1 : Symbol(j0.method1, Decl(extendPrivateConstructorClass2.ts, 78, 29))
+
+          method2() {}
+>method2 : Symbol(j0.method2, Decl(extendPrivateConstructorClass2.ts, 79, 22))
+        }
+      }
+    }
+  }
+}
+
+abstract class j9 {
+>j9 : Symbol(j9, Decl(extendPrivateConstructorClass2.ts, 85, 1))
+
+  private constructor(arg: string) {}
+>arg : Symbol(arg, Decl(extendPrivateConstructorClass2.ts, 88, 22))
+
+  method1() {
+>method1 : Symbol(j9.method1, Decl(extendPrivateConstructorClass2.ts, 88, 37))
+
+    abstract class j10 {
+>j10 : Symbol(j10, Decl(extendPrivateConstructorClass2.ts, 89, 13))
+
+      private constructor(arg: number) {}
+>arg : Symbol(arg, Decl(extendPrivateConstructorClass2.ts, 91, 26))
+
+      method2() {}
+>method2 : Symbol(j10.method2, Decl(extendPrivateConstructorClass2.ts, 91, 41))
+    }
+    const jS: typeof j9 & typeof j10 = null!;
+>jS : Symbol(jS, Decl(extendPrivateConstructorClass2.ts, 94, 9))
+>j9 : Symbol(j9, Decl(extendPrivateConstructorClass2.ts, 85, 1))
+>j10 : Symbol(j10, Decl(extendPrivateConstructorClass2.ts, 89, 13))
+
+    // error
+    class j0 extends jS {
+>j0 : Symbol(j0, Decl(extendPrivateConstructorClass2.ts, 94, 45))
+>jS : Symbol(jS, Decl(extendPrivateConstructorClass2.ts, 94, 9))
+
+      method1() {}
+>method1 : Symbol(j0.method1, Decl(extendPrivateConstructorClass2.ts, 97, 25))
+
+      method2() {}
+>method2 : Symbol(j0.method2, Decl(extendPrivateConstructorClass2.ts, 98, 18))
+    }
+  }
+}
+
+abstract class j11 {
+>j11 : Symbol(j11, Decl(extendPrivateConstructorClass2.ts, 102, 1))
+
+  private constructor(arg: string) {}
+>arg : Symbol(arg, Decl(extendPrivateConstructorClass2.ts, 105, 22))
+
+  static {
+    abstract class j12 {
+>j12 : Symbol(j12, Decl(extendPrivateConstructorClass2.ts, 106, 10))
+
+      private constructor(arg: number) {}
+>arg : Symbol(arg, Decl(extendPrivateConstructorClass2.ts, 108, 26))
+
+      static {
+        const jS: typeof j11 & typeof j12 = null!;
+>jS : Symbol(jS, Decl(extendPrivateConstructorClass2.ts, 110, 13))
+>j11 : Symbol(j11, Decl(extendPrivateConstructorClass2.ts, 102, 1))
+>j12 : Symbol(j12, Decl(extendPrivateConstructorClass2.ts, 106, 10))
+
+        // ok
+        class j0 extends jS {}
+>j0 : Symbol(j0, Decl(extendPrivateConstructorClass2.ts, 110, 50))
+>jS : Symbol(jS, Decl(extendPrivateConstructorClass2.ts, 110, 13))
+      }
+    }
+  }
+}
+
+abstract class j13 {
+>j13 : Symbol(j13, Decl(extendPrivateConstructorClass2.ts, 117, 1))
+
+  private constructor(arg: string) {}
+>arg : Symbol(arg, Decl(extendPrivateConstructorClass2.ts, 120, 22))
+
+  static {
+    abstract class j14 {
+>j14 : Symbol(j14, Decl(extendPrivateConstructorClass2.ts, 121, 10))
+
+      private constructor(arg: number) {}
+>arg : Symbol(arg, Decl(extendPrivateConstructorClass2.ts, 123, 26))
+    }
+    const jS: typeof j13 & typeof j14 = null!;
+>jS : Symbol(jS, Decl(extendPrivateConstructorClass2.ts, 125, 9))
+>j13 : Symbol(j13, Decl(extendPrivateConstructorClass2.ts, 117, 1))
+>j14 : Symbol(j14, Decl(extendPrivateConstructorClass2.ts, 121, 10))
+
+    // error
+    class j0 extends jS {}
+>j0 : Symbol(j0, Decl(extendPrivateConstructorClass2.ts, 125, 46))
+>jS : Symbol(jS, Decl(extendPrivateConstructorClass2.ts, 125, 9))
+  }
+}
+
diff --git a/tests/baselines/reference/extendPrivateConstructorClass2.types b/tests/baselines/reference/extendPrivateConstructorClass2.types
index 288803e0b0f75..96c1d3a6654ef 100644
--- a/tests/baselines/reference/extendPrivateConstructorClass2.types
+++ b/tests/baselines/reference/extendPrivateConstructorClass2.types
@@ -130,21 +130,19 @@ abstract class j3 {
 >args : any[]
 >     : ^^^^^
 
-      method2(_any: any) {
->method2 : (_any: any) => void
->        : ^    ^^   ^^^^^^^^^
->_any : any
->     : ^^^
+      method2() {
+>method2 : () => void
+>        : ^^^^^^^^^^
 
-        const jS: typeof j3 & typeof j4 = _any;
+        const jS: typeof j3 & typeof j4 = null!;
 >jS : typeof j3 & typeof j4
 >   : ^^^^^^^^^^^^^^^^^^^^^
 >j3 : typeof j3
 >   : ^^^^^^^^^
 >j4 : typeof j4
 >   : ^^^^^^^^^
->_any : any
->     : ^^^
+>null! : never
+>      : ^^^^^
 
         // bizarre but ok
         class j0 extends jS {
@@ -174,11 +172,9 @@ abstract class j5 {
 >args : any[]
 >     : ^^^^^
 
-  method1(_any: any) {
->method1 : (_any: any) => void
->        : ^    ^^   ^^^^^^^^^
->_any : any
->     : ^^^
+  method1() {
+>method1 : () => void
+>        : ^^^^^^^^^^
 
     abstract class j6 {
 >j6 : j6
@@ -192,15 +188,15 @@ abstract class j5 {
 >method2 : () => void
 >        : ^^^^^^^^^^
     }
-    const jS: typeof j5 & typeof j6 = _any;
+    const jS: typeof j5 & typeof j6 = null!;
 >jS : typeof j5 & typeof j6
 >   : ^^^^^^^^^^^^^^^^^^^^^
 >j5 : typeof j5
 >   : ^^^^^^^^^
 >j6 : typeof j6
 >   : ^^^^^^^^^
->_any : any
->     : ^^^
+>null! : never
+>      : ^^^^^
 
     // bizarre but ok too given the base is a result of a mixin
     class j0 extends jS {
@@ -220,3 +216,184 @@ abstract class j5 {
   }
 }
 
+abstract class j7 {
+>j7 : j7
+>   : ^^
+
+  private constructor(arg: string) {}
+>arg : string
+>    : ^^^^^^
+
+  method1() {
+>method1 : () => void
+>        : ^^^^^^^^^^
+
+    abstract class j8 {
+>j8 : j8
+>   : ^^
+
+      private constructor(arg: number) {}
+>arg : number
+>    : ^^^^^^
+
+      method2() {
+>method2 : () => void
+>        : ^^^^^^^^^^
+
+        const jS: typeof j7 & typeof j8 = null!;
+>jS : typeof j7 & typeof j8
+>   : ^^^^^^^^^^^^^^^^^^^^^
+>j7 : typeof j7
+>   : ^^^^^^^^^
+>j8 : typeof j8
+>   : ^^^^^^^^^
+>null! : never
+>      : ^^^^^
+
+        // error
+        class j0 extends jS {
+>j0 : j0
+>   : ^^
+>jS : j7
+>   : ^^
+
+          method1() {}
+>method1 : () => void
+>        : ^^^^^^^^^^
+
+          method2() {}
+>method2 : () => void
+>        : ^^^^^^^^^^
+        }
+      }
+    }
+  }
+}
+
+abstract class j9 {
+>j9 : j9
+>   : ^^
+
+  private constructor(arg: string) {}
+>arg : string
+>    : ^^^^^^
+
+  method1() {
+>method1 : () => void
+>        : ^^^^^^^^^^
+
+    abstract class j10 {
+>j10 : j10
+>    : ^^^
+
+      private constructor(arg: number) {}
+>arg : number
+>    : ^^^^^^
+
+      method2() {}
+>method2 : () => void
+>        : ^^^^^^^^^^
+    }
+    const jS: typeof j9 & typeof j10 = null!;
+>jS : typeof j9 & typeof j10
+>   : ^^^^^^^^^^^^^^^^^^^^^^
+>j9 : typeof j9
+>   : ^^^^^^^^^
+>j10 : typeof j10
+>    : ^^^^^^^^^^
+>null! : never
+>      : ^^^^^
+
+    // error
+    class j0 extends jS {
+>j0 : j0
+>   : ^^
+>jS : j9
+>   : ^^
+
+      method1() {}
+>method1 : () => void
+>        : ^^^^^^^^^^
+
+      method2() {}
+>method2 : () => void
+>        : ^^^^^^^^^^
+    }
+  }
+}
+
+abstract class j11 {
+>j11 : j11
+>    : ^^^
+
+  private constructor(arg: string) {}
+>arg : string
+>    : ^^^^^^
+
+  static {
+    abstract class j12 {
+>j12 : j12
+>    : ^^^
+
+      private constructor(arg: number) {}
+>arg : number
+>    : ^^^^^^
+
+      static {
+        const jS: typeof j11 & typeof j12 = null!;
+>jS : typeof j11 & typeof j12
+>   : ^^^^^^^^^^^^^^^^^^^^^^^
+>j11 : typeof j11
+>    : ^^^^^^^^^^
+>j12 : typeof j12
+>    : ^^^^^^^^^^
+>null! : never
+>      : ^^^^^
+
+        // ok
+        class j0 extends jS {}
+>j0 : j0
+>   : ^^
+>jS : j11
+>   : ^^^
+      }
+    }
+  }
+}
+
+abstract class j13 {
+>j13 : j13
+>    : ^^^
+
+  private constructor(arg: string) {}
+>arg : string
+>    : ^^^^^^
+
+  static {
+    abstract class j14 {
+>j14 : j14
+>    : ^^^
+
+      private constructor(arg: number) {}
+>arg : number
+>    : ^^^^^^
+    }
+    const jS: typeof j13 & typeof j14 = null!;
+>jS : typeof j13 & typeof j14
+>   : ^^^^^^^^^^^^^^^^^^^^^^^
+>j13 : typeof j13
+>    : ^^^^^^^^^^
+>j14 : typeof j14
+>    : ^^^^^^^^^^
+>null! : never
+>      : ^^^^^
+
+    // error
+    class j0 extends jS {}
+>j0 : j0
+>   : ^^
+>jS : j13
+>   : ^^^
+  }
+}
+
diff --git a/tests/cases/compiler/extendPrivateConstructorClass2.ts b/tests/cases/compiler/extendPrivateConstructorClass2.ts
index 9e060cbdb29f3..22b81c58ca466 100644
--- a/tests/cases/compiler/extendPrivateConstructorClass2.ts
+++ b/tests/cases/compiler/extendPrivateConstructorClass2.ts
@@ -40,8 +40,8 @@ abstract class j3 {
   method1() {
     abstract class j4 {
       private constructor(...args: any[]) {}
-      method2(_any: any) {
-        const jS: typeof j3 & typeof j4 = _any;
+      method2() {
+        const jS: typeof j3 & typeof j4 = null!;
 
         // bizarre but ok
         class j0 extends jS {
@@ -55,12 +55,12 @@ abstract class j3 {
 
 abstract class j5 {
   private constructor(...args: any[]) {}
-  method1(_any: any) {
+  method1() {
     abstract class j6 {
       private constructor(...args: any[]) {}
       method2() {}
     }
-    const jS: typeof j5 & typeof j6 = _any;
+    const jS: typeof j5 & typeof j6 = null!;
 
     // bizarre but ok too given the base is a result of a mixin
     class j0 extends jS {
@@ -69,3 +69,66 @@ abstract class j5 {
     }
   }
 }
+
+abstract class j7 {
+  private constructor(arg: string) {}
+  method1() {
+    abstract class j8 {
+      private constructor(arg: number) {}
+      method2() {
+        const jS: typeof j7 & typeof j8 = null!;
+
+        // error
+        class j0 extends jS {
+          method1() {}
+          method2() {}
+        }
+      }
+    }
+  }
+}
+
+abstract class j9 {
+  private constructor(arg: string) {}
+  method1() {
+    abstract class j10 {
+      private constructor(arg: number) {}
+      method2() {}
+    }
+    const jS: typeof j9 & typeof j10 = null!;
+
+    // error
+    class j0 extends jS {
+      method1() {}
+      method2() {}
+    }
+  }
+}
+
+abstract class j11 {
+  private constructor(arg: string) {}
+  static {
+    abstract class j12 {
+      private constructor(arg: number) {}
+      static {
+        const jS: typeof j11 & typeof j12 = null!;
+
+        // ok
+        class j0 extends jS {}
+      }
+    }
+  }
+}
+
+abstract class j13 {
+  private constructor(arg: string) {}
+  static {
+    abstract class j14 {
+      private constructor(arg: number) {}
+    }
+    const jS: typeof j13 & typeof j14 = null!;
+
+    // error
+    class j0 extends jS {}
+  }
+}

From 42cdf5955726278a5a672c5ad43763d3f53dd8fc Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Mateusz=20Burzy=C5=84ski?= <mateuszburzynski@gmail.com>
Date: Sat, 18 Oct 2025 09:16:52 +0200
Subject: [PATCH 6/6] fmt

---
 src/compiler/checker.ts | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index af7df91221824..e77aa179c7f7b 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -37171,7 +37171,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
                 return {
                     kind: modifiers,
                     declaringClass: getDeclaredTypeOfSymbol(declaration.parent.symbol),
-                }
+                };
             }
         }
         return undefined;