Fix #30408: Fallback to string index signature for literal and generic indexing

Ensure that string literal indexing and type parameters constrained to string
correctly fall back to an available string index signature if a specific
property is not found. This prevents confusing 'any' or 'missing property'
errors when a valid indexer exists.

Author: mhughes2012

src/compiler/checker.ts

@@ -19317,7 +19317,22 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
                                     const symbolName = getFullyQualifiedName((indexType as UniqueESSymbolType).symbol, accessExpression);
                                     errorInfo = chainDiagnosticMessages(/*details*/ undefined, Diagnostics.Property_0_does_not_exist_on_type_1, "[" + symbolName + "]", typeToString(objectType));
                                 }
-                                else if (indexType.flags & TypeFlags.StringLiteral) {
+
+                                // Handle literals, type parameters (like K extends string), and keyof types
+                                else if (indexType.flags & (TypeFlags.StringLiteral | TypeFlags.TypeParameter | TypeFlags.Index)) {
+
+                                    // 1. Try specific property (only for literals)
+                                    if (indexType.flags & TypeFlags.StringLiteral) {
+                                        const prop = getPropertyOfType(objectType, escapeLeadingUnderscores((indexType as StringLiteralType).value));
+                                        if (prop) return getTypeOfSymbol(prop);
+                                    }
+
+                                    // 2. Fallback to string index signature for anything string-like
+                                    if (isTypeAssignableTo(indexType, stringType)) {
+                                        const indexInfo = getIndexInfoOfType(objectType, stringType);
+                                        if (indexInfo) return indexInfo.type;
+                                    }
+
                                     errorInfo = chainDiagnosticMessages(/*details*/ undefined, Diagnostics.Property_0_does_not_exist_on_type_1, (indexType as StringLiteralType).value, typeToString(objectType));
                                 }
                                 else if (indexType.flags & TypeFlags.NumberLiteral) {

tests/baselines/reference/stringLiteralIndexingWithIndexSignature.js

@@ -0,0 +1,21 @@
+//// [tests/cases/compiler/stringLiteralIndexingWithIndexSignature.ts] ////
+
+//// [stringLiteralIndexingWithIndexSignature.ts]
+interface Dic<T> {
+    [key: string]: T;
+}
+
+function getVal<T, K extends string>(obj: Dic<T>, key: K) {
+    // This is the classic failure point for #30408
+    // It should resolve to 'T', but often resolves to 'any' or errors
+    return obj[key];
+}
+
+type Specific = Dic<number>["some_literal"]; // Should be 'number'
+
+//// [stringLiteralIndexingWithIndexSignature.js]
+function getVal(obj, key) {
+    // This is the classic failure point for #30408
+    // It should resolve to 'T', but often resolves to 'any' or errors
+    return obj[key];
+}

tests/baselines/reference/stringLiteralIndexingWithIndexSignature.symbols

@@ -0,0 +1,33 @@
+//// [tests/cases/compiler/stringLiteralIndexingWithIndexSignature.ts] ////
+
+=== stringLiteralIndexingWithIndexSignature.ts ===
+interface Dic<T> {
+>Dic : Symbol(Dic, Decl(stringLiteralIndexingWithIndexSignature.ts, 0, 0))
+>T : Symbol(T, Decl(stringLiteralIndexingWithIndexSignature.ts, 0, 14))
+
+    [key: string]: T;
+>key : Symbol(key, Decl(stringLiteralIndexingWithIndexSignature.ts, 1, 5))
+>T : Symbol(T, Decl(stringLiteralIndexingWithIndexSignature.ts, 0, 14))
+}
+
+function getVal<T, K extends string>(obj: Dic<T>, key: K) {
+>getVal : Symbol(getVal, Decl(stringLiteralIndexingWithIndexSignature.ts, 2, 1))
+>T : Symbol(T, Decl(stringLiteralIndexingWithIndexSignature.ts, 4, 16))
+>K : Symbol(K, Decl(stringLiteralIndexingWithIndexSignature.ts, 4, 18))
+>obj : Symbol(obj, Decl(stringLiteralIndexingWithIndexSignature.ts, 4, 37))
+>Dic : Symbol(Dic, Decl(stringLiteralIndexingWithIndexSignature.ts, 0, 0))
+>T : Symbol(T, Decl(stringLiteralIndexingWithIndexSignature.ts, 4, 16))
+>key : Symbol(key, Decl(stringLiteralIndexingWithIndexSignature.ts, 4, 49))
+>K : Symbol(K, Decl(stringLiteralIndexingWithIndexSignature.ts, 4, 18))
+
+    // This is the classic failure point for #30408
+    // It should resolve to 'T', but often resolves to 'any' or errors
+    return obj[key];
+>obj : Symbol(obj, Decl(stringLiteralIndexingWithIndexSignature.ts, 4, 37))
+>key : Symbol(key, Decl(stringLiteralIndexingWithIndexSignature.ts, 4, 49))
+}
+
+type Specific = Dic<number>["some_literal"]; // Should be 'number'
+>Specific : Symbol(Specific, Decl(stringLiteralIndexingWithIndexSignature.ts, 8, 1))
+>Dic : Symbol(Dic, Decl(stringLiteralIndexingWithIndexSignature.ts, 0, 0))
+

tests/baselines/reference/stringLiteralIndexingWithIndexSignature.types

@@ -0,0 +1,32 @@
+//// [tests/cases/compiler/stringLiteralIndexingWithIndexSignature.ts] ////
+
+=== stringLiteralIndexingWithIndexSignature.ts ===
+interface Dic<T> {
+    [key: string]: T;
+>key : string
+>    : ^^^^^^
+}
+
+function getVal<T, K extends string>(obj: Dic<T>, key: K) {
+>getVal : <T, K extends string>(obj: Dic<T>, key: K) => T
+>       : ^ ^^ ^^^^^^^^^      ^^   ^^      ^^   ^^ ^^^^^^
+>obj : Dic<T>
+>    : ^^^^^^
+>key : K
+>    : ^
+
+    // This is the classic failure point for #30408
+    // It should resolve to 'T', but often resolves to 'any' or errors
+    return obj[key];
+>obj[key] : T
+>         : ^
+>obj : Dic<T>
+>    : ^^^^^^
+>key : K
+>    : ^
+}
+
+type Specific = Dic<number>["some_literal"]; // Should be 'number'
+>Specific : number
+>         : ^^^^^^
+

tests/cases/compiler/stringLiteralIndexingWithIndexSignature.ts

@@ -0,0 +1,12 @@
+// @noImplicitAny: true
+interface Dic<T> {
+    [key: string]: T;
+}
+
+function getVal<T, K extends string>(obj: Dic<T>, key: K) {
+    // This is the classic failure point for #30408
+    // It should resolve to 'T', but often resolves to 'any' or errors
+    return obj[key];
+}
+
+type Specific = Dic<number>["some_literal"]; // Should be 'number'