Improve error message for index signature on generic type when writing (#55906)

src/compiler/checker.ts

@@ -17845,7 +17845,12 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
             if (indexInfo) {
                 if (accessFlags & AccessFlags.NoIndexSignatures && indexInfo.keyType !== numberType) {
                     if (accessExpression) {
-                        error(accessExpression, Diagnostics.Type_0_cannot_be_used_to_index_type_1, typeToString(indexType), typeToString(originalObjectType));
+                        if (accessFlags & AccessFlags.Writing) {
+                            error(accessExpression, Diagnostics.Type_0_is_generic_and_can_only_be_indexed_for_reading, typeToString(originalObjectType));
+                        }
+                        else {
+                            error(accessExpression, Diagnostics.Type_0_cannot_be_used_to_index_type_1, typeToString(indexType), typeToString(originalObjectType));
+                        }
                     }
                     return undefined;
                 }

src/compiler/diagnosticMessages.json

@@ -3699,6 +3699,10 @@
         "category": "Error",
         "code": 2861
     },
+    "Type '{0}' is generic and can only be indexed for reading.": {
+        "category": "Error",
+        "code": 2862
+    },
 
     "Import declaration '{0}' is using private name '{1}'.": {
         "category": "Error",

tests/baselines/reference/cannotIndexGenericWritingError.errors.txt

@@ -0,0 +1,19 @@
+cannotIndexGenericWritingError.ts(4,5): error TS2862: Type 'T' is generic and can only be indexed for reading.
+cannotIndexGenericWritingError.ts(8,5): error TS2862: Type 'T' is generic and can only be indexed for reading.
+
+
+==== cannotIndexGenericWritingError.ts (2 errors) ====
+    // From #47357
+    
+    function foo<T extends Record<string | symbol, any>>(target: T, p: string | symbol) {
+        target[p] = ""; // error
+        ~~~~~~~~~
+!!! error TS2862: Type 'T' is generic and can only be indexed for reading.
+    }
+    
+    function foo2<T extends number[] & { [s: string]: number | string }>(target: T, p: string | number) {
+        target[p] = 1; // error
+        ~~~~~~~~~
+!!! error TS2862: Type 'T' is generic and can only be indexed for reading.
+        target[1] = 1; // ok
+    }

tests/baselines/reference/cannotIndexGenericWritingError.symbols

@@ -0,0 +1,33 @@
+//// [tests/cases/compiler/cannotIndexGenericWritingError.ts] ////
+
+=== cannotIndexGenericWritingError.ts ===
+// From #47357
+
+function foo<T extends Record<string | symbol, any>>(target: T, p: string | symbol) {
+>foo : Symbol(foo, Decl(cannotIndexGenericWritingError.ts, 0, 0))
+>T : Symbol(T, Decl(cannotIndexGenericWritingError.ts, 2, 13))
+>Record : Symbol(Record, Decl(lib.es5.d.ts, --, --))
+>target : Symbol(target, Decl(cannotIndexGenericWritingError.ts, 2, 53))
+>T : Symbol(T, Decl(cannotIndexGenericWritingError.ts, 2, 13))
+>p : Symbol(p, Decl(cannotIndexGenericWritingError.ts, 2, 63))
+
+    target[p] = ""; // error
+>target : Symbol(target, Decl(cannotIndexGenericWritingError.ts, 2, 53))
+>p : Symbol(p, Decl(cannotIndexGenericWritingError.ts, 2, 63))
+}
+
+function foo2<T extends number[] & { [s: string]: number | string }>(target: T, p: string | number) {
+>foo2 : Symbol(foo2, Decl(cannotIndexGenericWritingError.ts, 4, 1))
+>T : Symbol(T, Decl(cannotIndexGenericWritingError.ts, 6, 14))
+>s : Symbol(s, Decl(cannotIndexGenericWritingError.ts, 6, 38))
+>target : Symbol(target, Decl(cannotIndexGenericWritingError.ts, 6, 69))
+>T : Symbol(T, Decl(cannotIndexGenericWritingError.ts, 6, 14))
+>p : Symbol(p, Decl(cannotIndexGenericWritingError.ts, 6, 79))
+
+    target[p] = 1; // error
+>target : Symbol(target, Decl(cannotIndexGenericWritingError.ts, 6, 69))
+>p : Symbol(p, Decl(cannotIndexGenericWritingError.ts, 6, 79))
+
+    target[1] = 1; // ok
+>target : Symbol(target, Decl(cannotIndexGenericWritingError.ts, 6, 69))
+}

tests/baselines/reference/cannotIndexGenericWritingError.types

@@ -0,0 +1,38 @@
+//// [tests/cases/compiler/cannotIndexGenericWritingError.ts] ////
+
+=== cannotIndexGenericWritingError.ts ===
+// From #47357
+
+function foo<T extends Record<string | symbol, any>>(target: T, p: string | symbol) {
+>foo : <T extends Record<string | symbol, any>>(target: T, p: string | symbol) => void
+>target : T
+>p : string | symbol
+
+    target[p] = ""; // error
+>target[p] = "" : ""
+>target[p] : any
+>target : T
+>p : string | symbol
+>"" : ""
+}
+
+function foo2<T extends number[] & { [s: string]: number | string }>(target: T, p: string | number) {
+>foo2 : <T extends number[] & { [s: string]: string | number; }>(target: T, p: string | number) => void
+>s : string
+>target : T
+>p : string | number
+
+    target[p] = 1; // error
+>target[p] = 1 : 1
+>target[p] : any
+>target : T
+>p : string | number
+>1 : 1
+
+    target[1] = 1; // ok
+>target[1] = 1 : 1
+>target[1] : number
+>target : T
+>1 : 1
+>1 : 1
+}

tests/baselines/reference/keyofAndIndexedAccess2.errors.txt

@@ -24,7 +24,7 @@ keyofAndIndexedAccess2.ts(52,3): error TS2322: Type 'number' is not assignable t
 keyofAndIndexedAccess2.ts(53,3): error TS2322: Type 'number' is not assignable to type 'T[K]'.
   'T[K]' could be instantiated with an arbitrary type which could be unrelated to 'number'.
 keyofAndIndexedAccess2.ts(65,7): error TS2339: Property 'foo' does not exist on type 'T'.
-keyofAndIndexedAccess2.ts(66,3): error TS2536: Type 'string' cannot be used to index type 'T'.
+keyofAndIndexedAccess2.ts(66,3): error TS2862: Type 'T' is generic and can only be indexed for reading.
 keyofAndIndexedAccess2.ts(67,3): error TS2322: Type 'number' is not assignable to type 'T[keyof T]'.
   'number' is assignable to the constraint of type 'T[keyof T]', but 'T[keyof T]' could be instantiated with a different subtype of constraint 'number'.
 keyofAndIndexedAccess2.ts(68,3): error TS2322: Type 'number' is not assignable to type 'T[K]'.
@@ -146,7 +146,7 @@ keyofAndIndexedAccess2.ts(108,5): error TS2322: Type '123' is not assignable to
 !!! error TS2339: Property 'foo' does not exist on type 'T'.
       obj[k1] = 123;  // Error
       ~~~~~~~
-!!! error TS2536: Type 'string' cannot be used to index type 'T'.
+!!! error TS2862: Type 'T' is generic and can only be indexed for reading.
       obj[k2] = 123;  // Error
       ~~~~~~~
 !!! error TS2322: Type 'number' is not assignable to type 'T[keyof T]'.

tests/baselines/reference/mappedTypeGenericWithKnownKeys.errors.txt

@@ -1,5 +1,5 @@
 mappedTypeGenericWithKnownKeys.ts(9,9): error TS2551: Property 'unknownLiteralKey' does not exist on type 'Record<keyof Shape | "knownLiteralKey", number>'. Did you mean 'knownLiteralKey'?
-mappedTypeGenericWithKnownKeys.ts(10,5): error TS2536: Type 'string' cannot be used to index type 'Record<keyof Shape | "knownLiteralKey", number>'.
+mappedTypeGenericWithKnownKeys.ts(10,5): error TS2862: Type 'Record<keyof Shape | "knownLiteralKey", number>' is generic and can only be indexed for reading.
 
 
 ==== mappedTypeGenericWithKnownKeys.ts (2 errors) ====
@@ -16,6 +16,6 @@ mappedTypeGenericWithKnownKeys.ts(10,5): error TS2536: Type 'string' cannot be u
 !!! error TS2551: Property 'unknownLiteralKey' does not exist on type 'Record<keyof Shape | "knownLiteralKey", number>'. Did you mean 'knownLiteralKey'?
         obj['' as string] = 4; // error
         ~~~~~~~~~~~~~~~~~
-!!! error TS2536: Type 'string' cannot be used to index type 'Record<keyof Shape | "knownLiteralKey", number>'.
+!!! error TS2862: Type 'Record<keyof Shape | "knownLiteralKey", number>' is generic and can only be indexed for reading.
     }
     

tests/cases/compiler/cannotIndexGenericWritingError.ts

@@ -0,0 +1,13 @@
+// @strict: true
+// @noEmit: true
+
+// From #47357
+
+function foo<T extends Record<string | symbol, any>>(target: T, p: string | symbol) {
+    target[p] = ""; // error
+}
+
+function foo2<T extends number[] & { [s: string]: number | string }>(target: T, p: string | number) {
+    target[p] = 1; // error
+    target[1] = 1; // ok
+}