fix(25770): add diagnostic message for the possible mapped type used …

…as an index

src/compiler/checker.ts

@@ -2246,16 +2246,28 @@ namespace ts {
                 }
                 const symbol = resolveSymbol(resolveName(errorLocation, name, SymbolFlags.Type & ~SymbolFlags.Value, /*nameNotFoundMessage*/undefined, /*nameArg*/ undefined, /*isUse*/ false));
                 if (symbol && !(symbol.flags & SymbolFlags.NamespaceModule)) {
-                    const message = isES2015OrLaterConstructorName(name)
-                        ? Diagnostics._0_only_refers_to_a_type_but_is_being_used_as_a_value_here_Do_you_need_to_change_your_target_library_Try_changing_the_lib_compiler_option_to_es2015_or_later
-                        : Diagnostics._0_only_refers_to_a_type_but_is_being_used_as_a_value_here;
+                    const message = isES2015OrLaterConstructorName(name) ?
+                        Diagnostics._0_only_refers_to_a_type_but_is_being_used_as_a_value_here_Do_you_need_to_change_your_target_library_Try_changing_the_lib_compiler_option_to_es2015_or_later :
+                        maybeMappedType(errorLocation, symbol) ?
+                            Diagnostics._0_only_refers_to_a_type_but_is_being_used_as_a_value_here_Did_you_mean_to_use_K_in_0 :
+                            Diagnostics._0_only_refers_to_a_type_but_is_being_used_as_a_value_here;
                     error(errorLocation, message, unescapeLeadingUnderscores(name));
                     return true;
                 }
             }
             return false;
         }
 
+        function maybeMappedType(node: Node, symbol: Symbol) {
+            const container = findAncestor(node.parent, n =>
+                isComputedPropertyName(n) || isPropertySignature(n) ? false : isTypeLiteralNode(n) || "quit") as TypeLiteralNode | undefined;
+            if (container && container.members.length === 1) {
+                const type = getDeclaredTypeOfSymbol(symbol);
+                return !!(type.flags & TypeFlags.Union) && allTypesAssignableToKind(type, TypeFlags.StringOrNumberLiteral, /*strict*/ true);
+            }
+            return false;
+        }
+
         function isES2015OrLaterConstructorName(n: __String) {
             switch (n) {
                 case "Promise":

src/compiler/diagnosticMessages.json

@@ -2607,6 +2607,10 @@
         "category": "Error",
         "code": 2689
     },
+    "'{0}' only refers to a type, but is being used as a value here. Did you mean to use 'K in {0}'?": {
+        "category": "Error",
+        "code": 2690
+    },
     "An import path cannot end with a '{0}' extension. Consider importing '{1}' instead.": {
         "category": "Error",
         "code": 2691

tests/baselines/reference/typeUsedAsTypeLiteralIndex.errors.txt

@@ -0,0 +1,54 @@
+tests/cases/compiler/typeUsedAsTypeLiteralIndex.ts(8,5): error TS1170: A computed property name in a type literal must refer to an expression whose type is a literal type or a 'unique symbol' type.
+tests/cases/compiler/typeUsedAsTypeLiteralIndex.ts(8,6): error TS2690: 'K2' only refers to a type, but is being used as a value here. Did you mean to use 'K in K2'?
+tests/cases/compiler/typeUsedAsTypeLiteralIndex.ts(11,6): error TS2300: Duplicate identifier 'K3'.
+tests/cases/compiler/typeUsedAsTypeLiteralIndex.ts(12,6): error TS2300: Duplicate identifier 'T3'.
+tests/cases/compiler/typeUsedAsTypeLiteralIndex.ts(13,5): error TS1170: A computed property name in a type literal must refer to an expression whose type is a literal type or a 'unique symbol' type.
+tests/cases/compiler/typeUsedAsTypeLiteralIndex.ts(13,6): error TS2690: 'K3' only refers to a type, but is being used as a value here. Did you mean to use 'K in K3'?
+tests/cases/compiler/typeUsedAsTypeLiteralIndex.ts(16,6): error TS2300: Duplicate identifier 'K3'.
+tests/cases/compiler/typeUsedAsTypeLiteralIndex.ts(17,6): error TS2300: Duplicate identifier 'T3'.
+tests/cases/compiler/typeUsedAsTypeLiteralIndex.ts(18,5): error TS1170: A computed property name in a type literal must refer to an expression whose type is a literal type or a 'unique symbol' type.
+tests/cases/compiler/typeUsedAsTypeLiteralIndex.ts(18,6): error TS2693: 'K3' only refers to a type, but is being used as a value here.
+
+
+==== tests/cases/compiler/typeUsedAsTypeLiteralIndex.ts (10 errors) ====
+    const K1 = Symbol();
+    type T1 = {
+        [K1]: number;
+    }
+
+    type K2 = "x" | "y";
+    type T2 = {
+        [K2]: number;
+        ~~~~
+!!! error TS1170: A computed property name in a type literal must refer to an expression whose type is a literal type or a 'unique symbol' type.
+         ~~
+!!! error TS2690: 'K2' only refers to a type, but is being used as a value here. Did you mean to use 'K in K2'?
+    }
+
+    type K3 = number | string;
+         ~~
+!!! error TS2300: Duplicate identifier 'K3'.
+    type T3 = {
+         ~~
+!!! error TS2300: Duplicate identifier 'T3'.
+        [K3]: number;
+        ~~~~
+!!! error TS1170: A computed property name in a type literal must refer to an expression whose type is a literal type or a 'unique symbol' type.
+         ~~
+!!! error TS2690: 'K3' only refers to a type, but is being used as a value here. Did you mean to use 'K in K3'?
+    }
+
+    type K3 = number | string;
+         ~~
+!!! error TS2300: Duplicate identifier 'K3'.
+    type T3 = {
+         ~~
+!!! error TS2300: Duplicate identifier 'T3'.
+        [K3]: number;
+        ~~~~
+!!! error TS1170: A computed property name in a type literal must refer to an expression whose type is a literal type or a 'unique symbol' type.
+         ~~
+!!! error TS2693: 'K3' only refers to a type, but is being used as a value here.
+        k4: string;
+    }
+

tests/baselines/reference/typeUsedAsTypeLiteralIndex.js

@@ -0,0 +1,25 @@
+//// [typeUsedAsTypeLiteralIndex.ts]
+const K1 = Symbol();
+type T1 = {
+    [K1]: number;
+}
+
+type K2 = "x" | "y";
+type T2 = {
+    [K2]: number;
+}
+
+type K3 = number | string;
+type T3 = {
+    [K3]: number;
+}
+
+type K3 = number | string;
+type T3 = {
+    [K3]: number;
+    k4: string;
+}
+
+
+//// [typeUsedAsTypeLiteralIndex.js]
+const K1 = Symbol();

tests/baselines/reference/typeUsedAsTypeLiteralIndex.symbols

@@ -0,0 +1,46 @@
+=== tests/cases/compiler/typeUsedAsTypeLiteralIndex.ts ===
+const K1 = Symbol();
+>K1 : Symbol(K1, Decl(typeUsedAsTypeLiteralIndex.ts, 0, 5))
+>Symbol : Symbol(Symbol, Decl(lib.es5.d.ts, --, --), Decl(lib.es2015.symbol.d.ts, --, --), Decl(lib.es2015.symbol.wellknown.d.ts, --, --), Decl(lib.es2019.symbol.d.ts, --, --))
+
+type T1 = {
+>T1 : Symbol(T1, Decl(typeUsedAsTypeLiteralIndex.ts, 0, 20))
+
+    [K1]: number;
+>[K1] : Symbol([K1], Decl(typeUsedAsTypeLiteralIndex.ts, 1, 11))
+>K1 : Symbol(K1, Decl(typeUsedAsTypeLiteralIndex.ts, 0, 5))
+}
+
+type K2 = "x" | "y";
+>K2 : Symbol(K2, Decl(typeUsedAsTypeLiteralIndex.ts, 3, 1))
+
+type T2 = {
+>T2 : Symbol(T2, Decl(typeUsedAsTypeLiteralIndex.ts, 5, 20))
+
+    [K2]: number;
+>[K2] : Symbol([K2], Decl(typeUsedAsTypeLiteralIndex.ts, 6, 11))
+}
+
+type K3 = number | string;
+>K3 : Symbol(K3, Decl(typeUsedAsTypeLiteralIndex.ts, 8, 1))
+
+type T3 = {
+>T3 : Symbol(T3, Decl(typeUsedAsTypeLiteralIndex.ts, 10, 26))
+
+    [K3]: number;
+>[K3] : Symbol([K3], Decl(typeUsedAsTypeLiteralIndex.ts, 11, 11))
+}
+
+type K3 = number | string;
+>K3 : Symbol(K3, Decl(typeUsedAsTypeLiteralIndex.ts, 13, 1))
+
+type T3 = {
+>T3 : Symbol(T3, Decl(typeUsedAsTypeLiteralIndex.ts, 15, 26))
+
+    [K3]: number;
+>[K3] : Symbol([K3], Decl(typeUsedAsTypeLiteralIndex.ts, 16, 11))
+
+    k4: string;
+>k4 : Symbol(k4, Decl(typeUsedAsTypeLiteralIndex.ts, 17, 17))
+}
+

tests/baselines/reference/typeUsedAsTypeLiteralIndex.types

@@ -0,0 +1,50 @@
+=== tests/cases/compiler/typeUsedAsTypeLiteralIndex.ts ===
+const K1 = Symbol();
+>K1 : unique symbol
+>Symbol() : unique symbol
+>Symbol : SymbolConstructor
+
+type T1 = {
+>T1 : T1
+
+    [K1]: number;
+>[K1] : number
+>K1 : unique symbol
+}
+
+type K2 = "x" | "y";
+>K2 : K2
+
+type T2 = {
+>T2 : T2
+
+    [K2]: number;
+>[K2] : number
+>K2 : any
+}
+
+type K3 = number | string;
+>K3 : K3
+
+type T3 = {
+>T3 : T3
+
+    [K3]: number;
+>[K3] : number
+>K3 : any
+}
+
+type K3 = number | string;
+>K3 : K3
+
+type T3 = {
+>T3 : { k4: string; }
+
+    [K3]: number;
+>[K3] : number
+>K3 : any
+
+    k4: string;
+>k4 : string
+}
+

tests/cases/compiler/typeUsedAsTypeLiteralIndex.ts

@@ -0,0 +1,22 @@
+// @target: esnext
+
+const K1 = Symbol();
+type T1 = {
+    [K1]: number;
+}
+
+type K2 = "x" | "y";
+type T2 = {
+    [K2]: number;
+}
+
+type K3 = number | string;
+type T3 = {
+    [K3]: number;
+}
+
+type K3 = number | string;
+type T3 = {
+    [K3]: number;
+    k4: string;
+}