Fix mixin logic to preserve at least one constructor type even when the

intersection also contains non-constructor types.

Fixes microsoft#17388.

Author: mattmccutchen

src/compiler/checker.ts

@@ -6639,13 +6639,23 @@ namespace ts {
                 getUnionType([info1.type, info2.type]), info1.isReadonly || info2.isReadonly);
         }
 
-        function includeMixinType(type: Type, types: ReadonlyArray<Type>, index: number): Type {
+        function findMixins(types: ReadonlyArray<Type>): ReadonlyArray<boolean> {
+            const constructorTypeCount = countWhere(types, (t) => getSignaturesOfType(t, SignatureKind.Construct).length > 0);
+            const mixinFlags = map(types, isMixinConstructorType);
+            if (constructorTypeCount > 0 && constructorTypeCount === countWhere(mixinFlags, (b) => b)) {
+                const firstMixinIndex = mixinFlags.indexOf(/*searchElement*/ true);
+                mixinFlags[firstMixinIndex] = false;
+            }
+            return mixinFlags;
+        }
+
+        function includeMixinType(type: Type, types: ReadonlyArray<Type>, mixinFlags: ReadonlyArray<boolean>, index: number): Type {
             const mixedTypes: Type[] = [];
             for (let i = 0; i < types.length; i++) {
                 if (i === index) {
                     mixedTypes.push(type);
                 }
-                else if (isMixinConstructorType(types[i])) {
+                else if (mixinFlags[i]) {
                     mixedTypes.push(getReturnTypeOfSignature(getSignaturesOfType(types[i], SignatureKind.Construct)[0]));
                 }
             }
@@ -6660,20 +6670,21 @@ namespace ts {
             let stringIndexInfo: IndexInfo | undefined;
             let numberIndexInfo: IndexInfo | undefined;
             const types = type.types;
-            const mixinCount = countWhere(types, isMixinConstructorType);
+            const mixinFlags = findMixins(types);
+            const mixinCount = countWhere(mixinFlags, (b) => b);
             for (let i = 0; i < types.length; i++) {
                 const t = type.types[i];
                 // When an intersection type contains mixin constructor types, the construct signatures from
                 // those types are discarded and their return types are mixed into the return types of all
                 // other construct signatures in the intersection type. For example, the intersection type
                 // '{ new(...args: any[]) => A } & { new(s: string) => B }' has a single construct signature
                 // 'new(s: string) => A & B'.
-                if (mixinCount === 0 || mixinCount === types.length && i === 0 || !isMixinConstructorType(t)) {
+                if (!mixinFlags[i]) {
                     let signatures = getSignaturesOfType(t, SignatureKind.Construct);
                     if (signatures.length && mixinCount > 0) {
                         signatures = map(signatures, s => {
                             const clone = cloneSignature(s);
-                            clone.resolvedReturnType = includeMixinType(getReturnTypeOfSignature(s), types, i);
+                            clone.resolvedReturnType = includeMixinType(getReturnTypeOfSignature(s), types, mixinFlags, i);
                             return clone;
                         });
                     }
@@ -20169,12 +20180,11 @@ namespace ts {
             const firstBase = baseTypes[0];
             if (firstBase.flags & TypeFlags.Intersection) {
                 const types = (firstBase as IntersectionType).types;
-                const mixinCount = countWhere(types, isMixinConstructorType);
+                const mixinFlags = findMixins(types);
                 let i = 0;
                 for (const intersectionMember of (firstBase as IntersectionType).types) {
-                    i++;
                     // We want to ignore mixin ctors
-                    if (mixinCount === 0 || mixinCount === types.length && i === 0 || !isMixinConstructorType(intersectionMember)) {
+                    if (!mixinFlags[i]) {
                         if (getObjectFlags(intersectionMember) & (ObjectFlags.Class | ObjectFlags.Interface)) {
                             if (intersectionMember.symbol === target) {
                                 return true;
@@ -20184,6 +20194,7 @@ namespace ts {
                             }
                         }
                     }
+                    i++;
                 }
                 return false;
             }

tests/baselines/reference/intersectionOfMixinConstructorTypeAndNonConstructorType.js

@@ -0,0 +1,10 @@
+//// [intersectionOfMixinConstructorTypeAndNonConstructorType.ts]
+// Repro for #17388
+
+declare let x: {foo: undefined} & {new(...args: any[]): any};
+new x();
+
+
+//// [intersectionOfMixinConstructorTypeAndNonConstructorType.js]
+// Repro for #17388
+new x();

tests/baselines/reference/intersectionOfMixinConstructorTypeAndNonConstructorType.symbols

@@ -0,0 +1,11 @@
+=== tests/cases/compiler/intersectionOfMixinConstructorTypeAndNonConstructorType.ts ===
+// Repro for #17388
+
+declare let x: {foo: undefined} & {new(...args: any[]): any};
+>x : Symbol(x, Decl(intersectionOfMixinConstructorTypeAndNonConstructorType.ts, 2, 11))
+>foo : Symbol(foo, Decl(intersectionOfMixinConstructorTypeAndNonConstructorType.ts, 2, 16))
+>args : Symbol(args, Decl(intersectionOfMixinConstructorTypeAndNonConstructorType.ts, 2, 39))
+
+new x();
+>x : Symbol(x, Decl(intersectionOfMixinConstructorTypeAndNonConstructorType.ts, 2, 11))
+

tests/baselines/reference/intersectionOfMixinConstructorTypeAndNonConstructorType.types

@@ -0,0 +1,12 @@
+=== tests/cases/compiler/intersectionOfMixinConstructorTypeAndNonConstructorType.ts ===
+// Repro for #17388
+
+declare let x: {foo: undefined} & {new(...args: any[]): any};
+>x : { foo: undefined; } & (new (...args: any[]) => any)
+>foo : undefined
+>args : any[]
+
+new x();
+>new x() : any
+>x : { foo: undefined; } & (new (...args: any[]) => any)
+

tests/cases/compiler/intersectionOfMixinConstructorTypeAndNonConstructorType.ts

@@ -0,0 +1,4 @@
+// Repro for #17388
+
+declare let x: {foo: undefined} & {new(...args: any[]): any};
+new x();