diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 645bf049f4e12..85273ece816c2 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -16494,13 +16494,40 @@ namespace ts {
                     const name = targetProp.escapedName;
                     if (!(targetProp.flags & SymbolFlags.Prototype) && (!numericNamesOnly || isNumericLiteralName(name) || name === "length")) {
                         const sourceProp = getPropertyOfType(source, name);
-                        if (sourceProp && sourceProp !== targetProp) {
+                        if (sourceProp === targetProp) {
+                            continue;
+                        }
+                        if (sourceProp) {
                             const related = propertyRelatedTo(source, target, sourceProp, targetProp, getTypeOfSymbol, reportErrors, intersectionState);
                             if (!related) {
                                 return Ternary.False;
                             }
                             result &= related;
                         }
+                        else if (!targetProp.valueDeclaration || !isPrivateIdentifierPropertyDeclaration(targetProp.valueDeclaration)) {
+                            Debug.assert(!!(targetProp.flags & SymbolFlags.Optional));
+                            let sourcePropType, diagnostic;
+                            if (isTupleType(source)) {
+                                sourcePropType = isNumericLiteralName(targetProp.escapedName) && +targetProp.escapedName >= 0 ? getRestTypeOfTupleType(source) : undefined;
+                                diagnostic = Diagnostics.Rest_element_type_is_incompatible_with_property_0;
+                            }
+                            else {
+                                sourcePropType =
+                                    isNumericLiteralName(targetProp.escapedName) && getIndexTypeOfType(source, IndexKind.Number) ||
+                                    getIndexTypeOfType(source, IndexKind.String);
+                                diagnostic = Diagnostics.Index_signature_is_incompatible_with_property_0;
+                            }
+                            if (sourcePropType) {
+                                const related = isRelatedTo(sourcePropType, getTypeOfSymbol(targetProp), reportErrors);
+                                if (!related) {
+                                    if (reportErrors) {
+                                        reportError(diagnostic, symbolToString(targetProp));
+                                    }
+                                    return Ternary.False;
+                                }
+                                result &= related;
+                            }
+                        }
                     }
                 }
                 return result;
diff --git a/src/compiler/commandLineParser.ts b/src/compiler/commandLineParser.ts
index 27f502a3f7de1..850825f5f230c 100644
--- a/src/compiler/commandLineParser.ts
+++ b/src/compiler/commandLineParser.ts
@@ -1196,7 +1196,7 @@ namespace ts {
     export function parseCommandLineWorker(
         diagnostics: ParseCommandLineWorkerDiagnostics,
         commandLine: readonly string[],
-        readFile?: (path: string) => string | undefined) {
+        readFile?: (path: string) => string | undefined): ParsedCommandLine {
         const options = {} as OptionsBase;
         let watchOptions: WatchOptions | undefined;
         const fileNames: string[] = [];
@@ -1204,7 +1204,7 @@ namespace ts {
 
         parseStrings(commandLine);
         return {
-            options,
+            options: options as CompilerOptions,
             watchOptions,
             fileNames,
             errors
diff --git a/src/compiler/diagnosticMessages.json b/src/compiler/diagnosticMessages.json
index c8bd3ceb7b79b..656a61c263765 100644
--- a/src/compiler/diagnosticMessages.json
+++ b/src/compiler/diagnosticMessages.json
@@ -2907,6 +2907,14 @@
         "category": "Error",
         "code": 2784
     },
+    "Index signature is incompatible with property '{0}'.": {
+        "category": "Error",
+        "code": 2785
+    },
+    "Rest element type is incompatible with property '{0}'.": {
+        "category": "Error",
+        "code": 2786
+    },
 
     "Import declaration '{0}' is using private name '{1}'.": {
         "category": "Error",
diff --git a/src/server/editorServices.ts b/src/server/editorServices.ts
index a79070dde80f7..3249bac6a8896 100644
--- a/src/server/editorServices.ts
+++ b/src/server/editorServices.ts
@@ -2641,7 +2641,7 @@ namespace ts.server {
                 }
 
                 if (args.watchOptions) {
-                    this.hostConfiguration.watchOptions = convertWatchOptions(args.watchOptions);
+                    this.hostConfiguration.watchOptions = convertWatchOptions(args.watchOptions as any);
                     this.logger.info(`Host watch options changed to ${JSON.stringify(this.hostConfiguration.watchOptions)}, it will be take effect for next watches.`);
                 }
             }
diff --git a/tests/baselines/reference/APISample_WatchWithOwnWatchHost.js b/tests/baselines/reference/APISample_WatchWithOwnWatchHost.js
index 75444a614f952..b9a7187eb0726 100644
--- a/tests/baselines/reference/APISample_WatchWithOwnWatchHost.js
+++ b/tests/baselines/reference/APISample_WatchWithOwnWatchHost.js
@@ -36,8 +36,8 @@ function watchMain() {
         readDirectory: ts.sys.readDirectory,
         realpath: ts.sys.realpath,
 
-        watchFile: ts.sys.watchFile!,
-        watchDirectory: ts.sys.watchDirectory!,
+        watchFile: ts.sys.watchFile! as any,
+        watchDirectory: ts.sys.watchDirectory! as any,
         createProgram: ts.createAbstractBuilder
     };
 
diff --git a/tests/baselines/reference/assignIndexSignatureToOptionalProperty.errors.txt b/tests/baselines/reference/assignIndexSignatureToOptionalProperty.errors.txt
new file mode 100644
index 0000000000000..4b0987542119c
--- /dev/null
+++ b/tests/baselines/reference/assignIndexSignatureToOptionalProperty.errors.txt
@@ -0,0 +1,30 @@
+tests/cases/compiler/assignIndexSignatureToOptionalProperty.ts(17,5): error TS2322: Type 'Bar' is not assignable to type 'Foo'.
+  Index signature is incompatible with property 'b'.
+    Type 'number' is not assignable to type 'string'.
+
+
+==== tests/cases/compiler/assignIndexSignatureToOptionalProperty.ts (1 errors) ====
+    // #27144
+    
+    interface Foo {
+        a: number;
+        b?: string;
+    }
+    interface Foo2 {
+        a: number;
+        b?: number;
+    }
+    interface Bar {
+        a: number;
+        [n: string]: number;
+    }
+    let b: Bar = { a: 42, b: 43 };
+    // Error, index signature does not match optional property `b`
+    let f: Foo = b;
+        ~
+!!! error TS2322: Type 'Bar' is not assignable to type 'Foo'.
+!!! error TS2322:   Index signature is incompatible with property 'b'.
+!!! error TS2322:     Type 'number' is not assignable to type 'string'.
+    // OK
+    let f2: Foo2 = b;
+    
\ No newline at end of file
diff --git a/tests/baselines/reference/assignIndexSignatureToOptionalProperty.js b/tests/baselines/reference/assignIndexSignatureToOptionalProperty.js
new file mode 100644
index 0000000000000..5289dc8a61275
--- /dev/null
+++ b/tests/baselines/reference/assignIndexSignatureToOptionalProperty.js
@@ -0,0 +1,29 @@
+//// [assignIndexSignatureToOptionalProperty.ts]
+// #27144
+
+interface Foo {
+    a: number;
+    b?: string;
+}
+interface Foo2 {
+    a: number;
+    b?: number;
+}
+interface Bar {
+    a: number;
+    [n: string]: number;
+}
+let b: Bar = { a: 42, b: 43 };
+// Error, index signature does not match optional property `b`
+let f: Foo = b;
+// OK
+let f2: Foo2 = b;
+
+
+//// [assignIndexSignatureToOptionalProperty.js]
+// #27144
+var b = { a: 42, b: 43 };
+// Error, index signature does not match optional property `b`
+var f = b;
+// OK
+var f2 = b;
diff --git a/tests/baselines/reference/assignIndexSignatureToOptionalProperty.symbols b/tests/baselines/reference/assignIndexSignatureToOptionalProperty.symbols
new file mode 100644
index 0000000000000..77c9e0e41e3f5
--- /dev/null
+++ b/tests/baselines/reference/assignIndexSignatureToOptionalProperty.symbols
@@ -0,0 +1,48 @@
+=== tests/cases/compiler/assignIndexSignatureToOptionalProperty.ts ===
+// #27144
+
+interface Foo {
+>Foo : Symbol(Foo, Decl(assignIndexSignatureToOptionalProperty.ts, 0, 0))
+
+    a: number;
+>a : Symbol(Foo.a, Decl(assignIndexSignatureToOptionalProperty.ts, 2, 15))
+
+    b?: string;
+>b : Symbol(Foo.b, Decl(assignIndexSignatureToOptionalProperty.ts, 3, 14))
+}
+interface Foo2 {
+>Foo2 : Symbol(Foo2, Decl(assignIndexSignatureToOptionalProperty.ts, 5, 1))
+
+    a: number;
+>a : Symbol(Foo2.a, Decl(assignIndexSignatureToOptionalProperty.ts, 6, 16))
+
+    b?: number;
+>b : Symbol(Foo2.b, Decl(assignIndexSignatureToOptionalProperty.ts, 7, 14))
+}
+interface Bar {
+>Bar : Symbol(Bar, Decl(assignIndexSignatureToOptionalProperty.ts, 9, 1))
+
+    a: number;
+>a : Symbol(Bar.a, Decl(assignIndexSignatureToOptionalProperty.ts, 10, 15))
+
+    [n: string]: number;
+>n : Symbol(n, Decl(assignIndexSignatureToOptionalProperty.ts, 12, 5))
+}
+let b: Bar = { a: 42, b: 43 };
+>b : Symbol(b, Decl(assignIndexSignatureToOptionalProperty.ts, 14, 3))
+>Bar : Symbol(Bar, Decl(assignIndexSignatureToOptionalProperty.ts, 9, 1))
+>a : Symbol(a, Decl(assignIndexSignatureToOptionalProperty.ts, 14, 14))
+>b : Symbol(b, Decl(assignIndexSignatureToOptionalProperty.ts, 14, 21))
+
+// Error, index signature does not match optional property `b`
+let f: Foo = b;
+>f : Symbol(f, Decl(assignIndexSignatureToOptionalProperty.ts, 16, 3))
+>Foo : Symbol(Foo, Decl(assignIndexSignatureToOptionalProperty.ts, 0, 0))
+>b : Symbol(b, Decl(assignIndexSignatureToOptionalProperty.ts, 14, 3))
+
+// OK
+let f2: Foo2 = b;
+>f2 : Symbol(f2, Decl(assignIndexSignatureToOptionalProperty.ts, 18, 3))
+>Foo2 : Symbol(Foo2, Decl(assignIndexSignatureToOptionalProperty.ts, 5, 1))
+>b : Symbol(b, Decl(assignIndexSignatureToOptionalProperty.ts, 14, 3))
+
diff --git a/tests/baselines/reference/assignIndexSignatureToOptionalProperty.types b/tests/baselines/reference/assignIndexSignatureToOptionalProperty.types
new file mode 100644
index 0000000000000..fce5e4952e201
--- /dev/null
+++ b/tests/baselines/reference/assignIndexSignatureToOptionalProperty.types
@@ -0,0 +1,42 @@
+=== tests/cases/compiler/assignIndexSignatureToOptionalProperty.ts ===
+// #27144
+
+interface Foo {
+    a: number;
+>a : number
+
+    b?: string;
+>b : string
+}
+interface Foo2 {
+    a: number;
+>a : number
+
+    b?: number;
+>b : number
+}
+interface Bar {
+    a: number;
+>a : number
+
+    [n: string]: number;
+>n : string
+}
+let b: Bar = { a: 42, b: 43 };
+>b : Bar
+>{ a: 42, b: 43 } : { a: number; b: number; }
+>a : number
+>42 : 42
+>b : number
+>43 : 43
+
+// Error, index signature does not match optional property `b`
+let f: Foo = b;
+>f : Foo
+>b : Bar
+
+// OK
+let f2: Foo2 = b;
+>f2 : Foo2
+>b : Bar
+
diff --git a/tests/baselines/reference/assignRestElementToOptionalProperty.errors.txt b/tests/baselines/reference/assignRestElementToOptionalProperty.errors.txt
new file mode 100644
index 0000000000000..7d7b81652c9a4
--- /dev/null
+++ b/tests/baselines/reference/assignRestElementToOptionalProperty.errors.txt
@@ -0,0 +1,18 @@
+tests/cases/compiler/assignRestElementToOptionalProperty.ts(5,5): error TS2322: Type '[number, ...string[]]' is not assignable to type '[number, number?, ...string[]]'.
+  Rest element type is incompatible with property '1'.
+    Type 'string' is not assignable to type 'number'.
+
+
+==== tests/cases/compiler/assignRestElementToOptionalProperty.ts (1 errors) ====
+    // Inspired by #27144
+    
+    let t: [number, ...string[]];
+    // Error, rest type of `t` does not match element 1 of `t2`
+    let t2: [number, number?, ...string[]] = t;
+        ~~
+!!! error TS2322: Type '[number, ...string[]]' is not assignable to type '[number, number?, ...string[]]'.
+!!! error TS2322:   Rest element type is incompatible with property '1'.
+!!! error TS2322:     Type 'string' is not assignable to type 'number'.
+    // OK
+    let t3: [number, string?, ...string[]] = t;
+    
\ No newline at end of file
diff --git a/tests/baselines/reference/assignRestElementToOptionalProperty.js b/tests/baselines/reference/assignRestElementToOptionalProperty.js
new file mode 100644
index 0000000000000..8500202a458e6
--- /dev/null
+++ b/tests/baselines/reference/assignRestElementToOptionalProperty.js
@@ -0,0 +1,17 @@
+//// [assignRestElementToOptionalProperty.ts]
+// Inspired by #27144
+
+let t: [number, ...string[]];
+// Error, rest type of `t` does not match element 1 of `t2`
+let t2: [number, number?, ...string[]] = t;
+// OK
+let t3: [number, string?, ...string[]] = t;
+
+
+//// [assignRestElementToOptionalProperty.js]
+// Inspired by #27144
+var t;
+// Error, rest type of `t` does not match element 1 of `t2`
+var t2 = t;
+// OK
+var t3 = t;
diff --git a/tests/baselines/reference/assignRestElementToOptionalProperty.symbols b/tests/baselines/reference/assignRestElementToOptionalProperty.symbols
new file mode 100644
index 0000000000000..dbfc353922ce8
--- /dev/null
+++ b/tests/baselines/reference/assignRestElementToOptionalProperty.symbols
@@ -0,0 +1,16 @@
+=== tests/cases/compiler/assignRestElementToOptionalProperty.ts ===
+// Inspired by #27144
+
+let t: [number, ...string[]];
+>t : Symbol(t, Decl(assignRestElementToOptionalProperty.ts, 2, 3))
+
+// Error, rest type of `t` does not match element 1 of `t2`
+let t2: [number, number?, ...string[]] = t;
+>t2 : Symbol(t2, Decl(assignRestElementToOptionalProperty.ts, 4, 3))
+>t : Symbol(t, Decl(assignRestElementToOptionalProperty.ts, 2, 3))
+
+// OK
+let t3: [number, string?, ...string[]] = t;
+>t3 : Symbol(t3, Decl(assignRestElementToOptionalProperty.ts, 6, 3))
+>t : Symbol(t, Decl(assignRestElementToOptionalProperty.ts, 2, 3))
+
diff --git a/tests/baselines/reference/assignRestElementToOptionalProperty.types b/tests/baselines/reference/assignRestElementToOptionalProperty.types
new file mode 100644
index 0000000000000..84033e50cb18b
--- /dev/null
+++ b/tests/baselines/reference/assignRestElementToOptionalProperty.types
@@ -0,0 +1,16 @@
+=== tests/cases/compiler/assignRestElementToOptionalProperty.ts ===
+// Inspired by #27144
+
+let t: [number, ...string[]];
+>t : [number, ...string[]]
+
+// Error, rest type of `t` does not match element 1 of `t2`
+let t2: [number, number?, ...string[]] = t;
+>t2 : [number, number?, ...string[]]
+>t : [number, ...string[]]
+
+// OK
+let t3: [number, string?, ...string[]] = t;
+>t3 : [number, string?, ...string[]]
+>t : [number, ...string[]]
+
diff --git a/tests/cases/compiler/APISample_WatchWithOwnWatchHost.ts b/tests/cases/compiler/APISample_WatchWithOwnWatchHost.ts
index 18cb5766ce331..c7f9963a1f8d7 100644
--- a/tests/cases/compiler/APISample_WatchWithOwnWatchHost.ts
+++ b/tests/cases/compiler/APISample_WatchWithOwnWatchHost.ts
@@ -40,8 +40,8 @@ function watchMain() {
         readDirectory: ts.sys.readDirectory,
         realpath: ts.sys.realpath,
 
-        watchFile: ts.sys.watchFile!,
-        watchDirectory: ts.sys.watchDirectory!,
+        watchFile: ts.sys.watchFile! as any,
+        watchDirectory: ts.sys.watchDirectory! as any,
         createProgram: ts.createAbstractBuilder
     };
 
diff --git a/tests/cases/compiler/assignIndexSignatureToOptionalProperty.ts b/tests/cases/compiler/assignIndexSignatureToOptionalProperty.ts
new file mode 100644
index 0000000000000..26832ad9a5762
--- /dev/null
+++ b/tests/cases/compiler/assignIndexSignatureToOptionalProperty.ts
@@ -0,0 +1,19 @@
+// #27144
+
+interface Foo {
+    a: number;
+    b?: string;
+}
+interface Foo2 {
+    a: number;
+    b?: number;
+}
+interface Bar {
+    a: number;
+    [n: string]: number;
+}
+let b: Bar = { a: 42, b: 43 };
+// Error, index signature does not match optional property `b`
+let f: Foo = b;
+// OK
+let f2: Foo2 = b;
diff --git a/tests/cases/compiler/assignRestElementToOptionalProperty.ts b/tests/cases/compiler/assignRestElementToOptionalProperty.ts
new file mode 100644
index 0000000000000..0490db5088e06
--- /dev/null
+++ b/tests/cases/compiler/assignRestElementToOptionalProperty.ts
@@ -0,0 +1,7 @@
+// Inspired by #27144
+
+let t: [number, ...string[]];
+// Error, rest type of `t` does not match element 1 of `t2`
+let t2: [number, number?, ...string[]] = t;
+// OK
+let t3: [number, string?, ...string[]] = t;