Relax requirements on index signatures to 'any' when a type also contains a string index signature to 'any'

src/compiler/checker.ts

@@ -18948,10 +18948,18 @@ namespace ts {
                     return indexTypesIdenticalTo(source, target, kind);
                 }
                 const targetType = getIndexTypeOfType(target, kind);
-                if (!targetType || targetType.flags & TypeFlags.Any && !sourceIsPrimitive && kind === IndexKind.String) {
-                    // Index signature of type any permits assignment from everything but primitives
+                if (!targetType) {
                     return Ternary.True;
                 }
+                if (targetType.flags & TypeFlags.Any && !sourceIsPrimitive) {
+                    // An index signature of type `any` permits assignment from everything but primitives,
+                    // provided that there is also a `string` index signature of type `any`.
+                    const stringIndexType = kind === IndexKind.String ? targetType : getIndexTypeOfType(target, IndexKind.String);
+                    if (stringIndexType && stringIndexType.flags & TypeFlags.Any) {
+                        return Ternary.True;
+                    }
+
+                }
                 if (isGenericMappedType(source)) {
                     // A generic mapped type { [P in K]: T } is related to a type with an index signature
                     // { [x: string]: U }, and optionally with an index signature { [x: number]: V },

tests/baselines/reference/objectTypeWithStringAndNumberIndexSignatureToAny.errors.txt

@@ -0,0 +1,145 @@
+tests/cases/conformance/types/members/objectTypeWithStringAndNumberIndexSignatureToAny.ts(43,5): error TS2322: Type 'Obj' is not assignable to type 'NumberTo<any>'.
+  Index signature is missing in type 'Obj'.
+tests/cases/conformance/types/members/objectTypeWithStringAndNumberIndexSignatureToAny.ts(49,5): error TS2739: Type 'StringTo<any>' is missing the following properties from type 'Obj': hello, world
+tests/cases/conformance/types/members/objectTypeWithStringAndNumberIndexSignatureToAny.ts(50,5): error TS2739: Type 'NumberTo<any>' is missing the following properties from type 'Obj': hello, world
+tests/cases/conformance/types/members/objectTypeWithStringAndNumberIndexSignatureToAny.ts(51,5): error TS2739: Type 'StringAndNumberTo<any>' is missing the following properties from type 'Obj': hello, world
+tests/cases/conformance/types/members/objectTypeWithStringAndNumberIndexSignatureToAny.ts(61,5): error TS2322: Type 'Obj' is not assignable to type 'NumberTo<any>'.
+tests/cases/conformance/types/members/objectTypeWithStringAndNumberIndexSignatureToAny.ts(65,5): error TS2322: Type 'Obj' is not assignable to type 'StringTo<any> & NumberTo<any>'.
+  Type 'Obj' is not assignable to type 'NumberTo<any>'.
+    Index signature is missing in type 'Obj'.
+tests/cases/conformance/types/members/objectTypeWithStringAndNumberIndexSignatureToAny.ts(67,5): error TS2322: Type 'StringTo<any>' is not assignable to type 'Obj'.
+tests/cases/conformance/types/members/objectTypeWithStringAndNumberIndexSignatureToAny.ts(68,5): error TS2322: Type 'NumberTo<any>' is not assignable to type 'Obj'.
+tests/cases/conformance/types/members/objectTypeWithStringAndNumberIndexSignatureToAny.ts(69,5): error TS2739: Type 'StringTo<any> & NumberTo<any>' is missing the following properties from type 'Obj': hello, world
+tests/cases/conformance/types/members/objectTypeWithStringAndNumberIndexSignatureToAny.ts(84,5): error TS2322: Type 'Obj' is not assignable to type 'NumberToNumber'.
+  Index signature is missing in type 'Obj'.
+tests/cases/conformance/types/members/objectTypeWithStringAndNumberIndexSignatureToAny.ts(88,5): error TS2322: Type 'Obj' is not assignable to type 'StringToAnyNumberToNumber'.
+  Index signature is missing in type 'Obj'.
+tests/cases/conformance/types/members/objectTypeWithStringAndNumberIndexSignatureToAny.ts(90,5): error TS2322: Type 'StringTo<any>' is not assignable to type 'Obj'.
+tests/cases/conformance/types/members/objectTypeWithStringAndNumberIndexSignatureToAny.ts(91,5): error TS2739: Type 'NumberTo<number>' is missing the following properties from type 'Obj': hello, world
+
+
+==== tests/cases/conformance/types/members/objectTypeWithStringAndNumberIndexSignatureToAny.ts (13 errors) ====
+    // When checking compatibility between two types,
+    // TypeScript should not require an index signature if
+    // the target side index signature maps to `any` *and*
+    // the target side has *any* string index signature to `any`.
+    //
+    // So an index signature like in
+    //
+    //  { [x: number]: any }
+    //
+    // is still required of a source type, but neither index signature in
+    //
+    //  { [x: number]: any, [x: string]: any; }
+    //
+    // should be required; *however*, the number index signature in
+    //
+    //  { [x: number]: number, [x: string]: any; }
+    //
+    // should always be required.
+
+    interface StringTo<T> {
+        [x: string]: T;
+    }
+
+    interface NumberTo<T> {
+        [x: number]: T;
+    }
+
+    interface StringAndNumberTo<T> extends StringTo<T>, NumberTo<T> {
+    }
+
+    interface Obj {
+        hello: string;
+        world: number;
+    }
+
+    function f1(sToAny: StringTo<any>, nToAny: NumberTo<any>, bothToAny: StringAndNumberTo<any>, someObj: Obj) {
+        sToAny = nToAny;
+        sToAny = bothToAny;
+        sToAny = someObj;
+
+        nToAny = sToAny;
+        nToAny = bothToAny;
+        nToAny = someObj;
+        ~~~~~~
+!!! error TS2322: Type 'Obj' is not assignable to type 'NumberTo<any>'.
+!!! error TS2322:   Index signature is missing in type 'Obj'.
+
+        bothToAny = sToAny;
+        bothToAny = nToAny;
+        bothToAny = someObj;
+
+        someObj = sToAny;
+        ~~~~~~~
+!!! error TS2739: Type 'StringTo<any>' is missing the following properties from type 'Obj': hello, world
+        someObj = nToAny;
+        ~~~~~~~
+!!! error TS2739: Type 'NumberTo<any>' is missing the following properties from type 'Obj': hello, world
+        someObj = bothToAny;
+        ~~~~~~~
+!!! error TS2739: Type 'StringAndNumberTo<any>' is missing the following properties from type 'Obj': hello, world
+    }
+
+    function f2(sToAny: StringTo<any>, nToAny: NumberTo<any>, bothToAny: StringTo<any> & NumberTo<any>, someObj: Obj) {
+        sToAny = nToAny;
+        sToAny = bothToAny;
+        sToAny = someObj;
+
+        nToAny = sToAny;
+        nToAny = bothToAny;
+        nToAny = someObj;
+        ~~~~~~
+!!! error TS2322: Type 'Obj' is not assignable to type 'NumberTo<any>'.
+
+        bothToAny = sToAny;
+        bothToAny = nToAny;
+        bothToAny = someObj;
+        ~~~~~~~~~
+!!! error TS2322: Type 'Obj' is not assignable to type 'StringTo<any> & NumberTo<any>'.
+!!! error TS2322:   Type 'Obj' is not assignable to type 'NumberTo<any>'.
+!!! error TS2322:     Index signature is missing in type 'Obj'.
+
+        someObj = sToAny;
+        ~~~~~~~
+!!! error TS2322: Type 'StringTo<any>' is not assignable to type 'Obj'.
+        someObj = nToAny;
+        ~~~~~~~
+!!! error TS2322: Type 'NumberTo<any>' is not assignable to type 'Obj'.
+        someObj = bothToAny;
+        ~~~~~~~
+!!! error TS2739: Type 'StringTo<any> & NumberTo<any>' is missing the following properties from type 'Obj': hello, world
+    }
+
+    type NumberToNumber = NumberTo<number>;
+
+    interface StringToAnyNumberToNumber extends StringTo<any>, NumberToNumber {
+    }
+
+    function f3(sToAny: StringTo<any>, nToNumber: NumberToNumber, strToAnyNumToNum: StringToAnyNumberToNumber, someObj: Obj) {
+        sToAny = nToNumber;
+        sToAny = strToAnyNumToNum;
+        sToAny = someObj;
+
+        nToNumber = sToAny;
+        nToNumber = strToAnyNumToNum;
+        nToNumber = someObj;
+        ~~~~~~~~~
+!!! error TS2322: Type 'Obj' is not assignable to type 'NumberToNumber'.
+!!! error TS2322:   Index signature is missing in type 'Obj'.
+
+        strToAnyNumToNum = sToAny;
+        strToAnyNumToNum = nToNumber;
+        strToAnyNumToNum = someObj;
+        ~~~~~~~~~~~~~~~~
+!!! error TS2322: Type 'Obj' is not assignable to type 'StringToAnyNumberToNumber'.
+!!! error TS2322:   Index signature is missing in type 'Obj'.
+
+        someObj = sToAny;
+        ~~~~~~~
+!!! error TS2322: Type 'StringTo<any>' is not assignable to type 'Obj'.
+        someObj = nToNumber;
+        ~~~~~~~
+!!! error TS2739: Type 'NumberTo<number>' is missing the following properties from type 'Obj': hello, world
+        someObj = someObj;
+    }

tests/baselines/reference/objectTypeWithStringAndNumberIndexSignatureToAny.js

@@ -0,0 +1,157 @@
+//// [objectTypeWithStringAndNumberIndexSignatureToAny.ts]
+// When checking compatibility between two types,
+// TypeScript should not require an index signature if
+// the target side index signature maps to `any` *and*
+// the target side has *any* string index signature to `any`.
+//
+// So an index signature like in
+//
+//  { [x: number]: any }
+//
+// is still required of a source type, but neither index signature in
+//
+//  { [x: number]: any, [x: string]: any; }
+//
+// should be required; *however*, the number index signature in
+//
+//  { [x: number]: number, [x: string]: any; }
+//
+// should always be required.
+
+interface StringTo<T> {
+    [x: string]: T;
+}
+
+interface NumberTo<T> {
+    [x: number]: T;
+}
+
+interface StringAndNumberTo<T> extends StringTo<T>, NumberTo<T> {
+}
+
+interface Obj {
+    hello: string;
+    world: number;
+}
+
+function f1(sToAny: StringTo<any>, nToAny: NumberTo<any>, bothToAny: StringAndNumberTo<any>, someObj: Obj) {
+    sToAny = nToAny;
+    sToAny = bothToAny;
+    sToAny = someObj;
+
+    nToAny = sToAny;
+    nToAny = bothToAny;
+    nToAny = someObj;
+
+    bothToAny = sToAny;
+    bothToAny = nToAny;
+    bothToAny = someObj;
+
+    someObj = sToAny;
+    someObj = nToAny;
+    someObj = bothToAny;
+}
+
+function f2(sToAny: StringTo<any>, nToAny: NumberTo<any>, bothToAny: StringTo<any> & NumberTo<any>, someObj: Obj) {
+    sToAny = nToAny;
+    sToAny = bothToAny;
+    sToAny = someObj;
+
+    nToAny = sToAny;
+    nToAny = bothToAny;
+    nToAny = someObj;
+
+    bothToAny = sToAny;
+    bothToAny = nToAny;
+    bothToAny = someObj;
+
+    someObj = sToAny;
+    someObj = nToAny;
+    someObj = bothToAny;
+}
+
+type NumberToNumber = NumberTo<number>;
+
+interface StringToAnyNumberToNumber extends StringTo<any>, NumberToNumber {
+}
+
+function f3(sToAny: StringTo<any>, nToNumber: NumberToNumber, strToAnyNumToNum: StringToAnyNumberToNumber, someObj: Obj) {
+    sToAny = nToNumber;
+    sToAny = strToAnyNumToNum;
+    sToAny = someObj;
+
+    nToNumber = sToAny;
+    nToNumber = strToAnyNumToNum;
+    nToNumber = someObj;
+
+    strToAnyNumToNum = sToAny;
+    strToAnyNumToNum = nToNumber;
+    strToAnyNumToNum = someObj;
+
+    someObj = sToAny;
+    someObj = nToNumber;
+    someObj = someObj;
+}
+
+//// [objectTypeWithStringAndNumberIndexSignatureToAny.js]
+"use strict";
+// When checking compatibility between two types,
+// TypeScript should not require an index signature if
+// the target side index signature maps to `any` *and*
+// the target side has *any* string index signature to `any`.
+//
+// So an index signature like in
+//
+//  { [x: number]: any }
+//
+// is still required of a source type, but neither index signature in
+//
+//  { [x: number]: any, [x: string]: any; }
+//
+// should be required; *however*, the number index signature in
+//
+//  { [x: number]: number, [x: string]: any; }
+//
+// should always be required.
+function f1(sToAny, nToAny, bothToAny, someObj) {
+    sToAny = nToAny;
+    sToAny = bothToAny;
+    sToAny = someObj;
+    nToAny = sToAny;
+    nToAny = bothToAny;
+    nToAny = someObj;
+    bothToAny = sToAny;
+    bothToAny = nToAny;
+    bothToAny = someObj;
+    someObj = sToAny;
+    someObj = nToAny;
+    someObj = bothToAny;
+}
+function f2(sToAny, nToAny, bothToAny, someObj) {
+    sToAny = nToAny;
+    sToAny = bothToAny;
+    sToAny = someObj;
+    nToAny = sToAny;
+    nToAny = bothToAny;
+    nToAny = someObj;
+    bothToAny = sToAny;
+    bothToAny = nToAny;
+    bothToAny = someObj;
+    someObj = sToAny;
+    someObj = nToAny;
+    someObj = bothToAny;
+}
+function f3(sToAny, nToNumber, strToAnyNumToNum, someObj) {
+    sToAny = nToNumber;
+    sToAny = strToAnyNumToNum;
+    sToAny = someObj;
+    nToNumber = sToAny;
+    nToNumber = strToAnyNumToNum;
+    nToNumber = someObj;
+    strToAnyNumToNum = sToAny;
+    strToAnyNumToNum = nToNumber;
+    strToAnyNumToNum = someObj;
+    someObj = sToAny;
+    someObj = nToNumber;
+    someObj = someObj;
+}