Apply (Un)Capitalize template string mappings on generic template literal types in a safe manner

src/compiler/checker.ts

@@ -17196,7 +17196,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
     function getStringMappingType(symbol: Symbol, type: Type): Type {
         return type.flags & (TypeFlags.Union | TypeFlags.Never) ? mapType(type, t => getStringMappingType(symbol, t)) :
             type.flags & TypeFlags.StringLiteral ? getStringLiteralType(applyStringMapping(symbol, (type as StringLiteralType).value)) :
-            type.flags & TypeFlags.TemplateLiteral ? getTemplateLiteralType(...applyTemplateStringMapping(symbol, (type as TemplateLiteralType).texts, (type as TemplateLiteralType).types)) :
+            type.flags & TypeFlags.TemplateLiteral ? applyTemplateStringMapping(symbol, type as TemplateLiteralType) :
             // Mapping<Mapping<T>> === Mapping<T>
             type.flags & TypeFlags.StringMapping && symbol === type.symbol ? type :
             type.flags & (TypeFlags.Any | TypeFlags.String | TypeFlags.StringMapping) || isGenericIndexType(type) ? getStringMappingTypeForGenericType(symbol, type) :
@@ -17215,14 +17215,23 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
         return str;
     }
 
-    function applyTemplateStringMapping(symbol: Symbol, texts: readonly string[], types: readonly Type[]): [texts: readonly string[], types: readonly Type[]] {
+    function applyTemplateStringMapping(symbol: Symbol, type: TemplateLiteralType): Type {
+        const { texts, types } = type;
         switch (intrinsicTypeKinds.get(symbol.escapedName as string)) {
-            case IntrinsicTypeKind.Uppercase: return [texts.map(t => t.toUpperCase()), types.map(t => getStringMappingType(symbol, t))];
-            case IntrinsicTypeKind.Lowercase: return [texts.map(t => t.toLowerCase()), types.map(t => getStringMappingType(symbol, t))];
-            case IntrinsicTypeKind.Capitalize: return [texts[0] === "" ? texts : [texts[0].charAt(0).toUpperCase() + texts[0].slice(1), ...texts.slice(1)], texts[0] === "" ? [getStringMappingType(symbol, types[0]), ...types.slice(1)] : types];
-            case IntrinsicTypeKind.Uncapitalize: return [texts[0] === "" ? texts : [texts[0].charAt(0).toLowerCase() + texts[0].slice(1), ...texts.slice(1)], texts[0] === "" ? [getStringMappingType(symbol, types[0]), ...types.slice(1)] : types];
+            case IntrinsicTypeKind.Uppercase: return getTemplateLiteralType(texts.map(t => t.toUpperCase()), types.map(t => getStringMappingType(symbol, t)));
+            case IntrinsicTypeKind.Lowercase: return getTemplateLiteralType(texts.map(t => t.toLowerCase()), types.map(t => getStringMappingType(symbol, t)));
+            case IntrinsicTypeKind.Capitalize:
+            case IntrinsicTypeKind.Uncapitalize:
+                return texts[0] === "" && types.length
+                    ? types.length === 1 && texts[1] === ""
+                        ? getStringMappingType(symbol, types[0])
+                        : getStringMappingTypeForGenericType(symbol, type)
+                    : getTemplateLiteralType(
+                        [applyStringMapping(symbol, texts[0]), ...texts.slice(1)],
+                        types
+                    );
         }
-        return [texts, types];
+        return getTemplateLiteralType(texts, types);
     }
 
     function getStringMappingTypeForGenericType(symbol: Symbol, type: Type): Type {

tests/baselines/reference/genericStringMappings.symbols

@@ -0,0 +1,81 @@
+//// [tests/cases/conformance/types/literal/genericStringMappings.ts] ////
+
+=== genericStringMappings.ts ===
+// repro from #52102#issuecomment-1371248589
+
+type CamelCase1<
+>CamelCase1 : Symbol(CamelCase1, Decl(genericStringMappings.ts, 0, 0))
+
+  S extends string,
+>S : Symbol(S, Decl(genericStringMappings.ts, 2, 16))
+
+  L extends string = Lowercase<S>,
+>L : Symbol(L, Decl(genericStringMappings.ts, 3, 19))
+>Lowercase : Symbol(Lowercase, Decl(lib.es5.d.ts, --, --))
+>S : Symbol(S, Decl(genericStringMappings.ts, 2, 16))
+
+  Res extends string = ""
+>Res : Symbol(Res, Decl(genericStringMappings.ts, 4, 34))
+
+> = L extends ""
+>L : Symbol(L, Decl(genericStringMappings.ts, 3, 19))
+
+  ? `start_${Uncapitalize<Res>}_end`
+>Uncapitalize : Symbol(Uncapitalize, Decl(lib.es5.d.ts, --, --))
+>Res : Symbol(Res, Decl(genericStringMappings.ts, 4, 34))
+
+  : CamelCase1<never, "", `${Res}${Capitalize<L>}`>;
+>CamelCase1 : Symbol(CamelCase1, Decl(genericStringMappings.ts, 0, 0))
+>Res : Symbol(Res, Decl(genericStringMappings.ts, 4, 34))
+>Capitalize : Symbol(Capitalize, Decl(lib.es5.d.ts, --, --))
+>L : Symbol(L, Decl(genericStringMappings.ts, 3, 19))
+
+type Test1 = CamelCase1<"ABC">
+>Test1 : Symbol(Test1, Decl(genericStringMappings.ts, 8, 52))
+>CamelCase1 : Symbol(CamelCase1, Decl(genericStringMappings.ts, 0, 0))
+
+// repro from #52102
+
+type CamelCase2<
+>CamelCase2 : Symbol(CamelCase2, Decl(genericStringMappings.ts, 10, 30))
+
+  S extends string,
+>S : Symbol(S, Decl(genericStringMappings.ts, 14, 16))
+
+  L extends string = Lowercase<S>,
+>L : Symbol(L, Decl(genericStringMappings.ts, 15, 19))
+>Lowercase : Symbol(Lowercase, Decl(lib.es5.d.ts, --, --))
+>S : Symbol(S, Decl(genericStringMappings.ts, 14, 16))
+
+  Res extends string = ""
+>Res : Symbol(Res, Decl(genericStringMappings.ts, 16, 34))
+
+> = L extends ""
+>L : Symbol(L, Decl(genericStringMappings.ts, 15, 19))
+
+  ? Uncapitalize<Res>
+>Uncapitalize : Symbol(Uncapitalize, Decl(lib.es5.d.ts, --, --))
+>Res : Symbol(Res, Decl(genericStringMappings.ts, 16, 34))
+
+  : L extends `${infer H}_${infer T}`
+>L : Symbol(L, Decl(genericStringMappings.ts, 15, 19))
+>H : Symbol(H, Decl(genericStringMappings.ts, 20, 22))
+>T : Symbol(T, Decl(genericStringMappings.ts, 20, 33))
+
+  ? CamelCase2<never, T, `${Res}${Capitalize<H>}`>
+>CamelCase2 : Symbol(CamelCase2, Decl(genericStringMappings.ts, 10, 30))
+>T : Symbol(T, Decl(genericStringMappings.ts, 20, 33))
+>Res : Symbol(Res, Decl(genericStringMappings.ts, 16, 34))
+>Capitalize : Symbol(Capitalize, Decl(lib.es5.d.ts, --, --))
+>H : Symbol(H, Decl(genericStringMappings.ts, 20, 22))
+
+  : CamelCase2<never, "", `${Res}${Capitalize<L>}`>;
+>CamelCase2 : Symbol(CamelCase2, Decl(genericStringMappings.ts, 10, 30))
+>Res : Symbol(Res, Decl(genericStringMappings.ts, 16, 34))
+>Capitalize : Symbol(Capitalize, Decl(lib.es5.d.ts, --, --))
+>L : Symbol(L, Decl(genericStringMappings.ts, 15, 19))
+
+type Test2 = CamelCase2<"FOOBAR">
+>Test2 : Symbol(Test2, Decl(genericStringMappings.ts, 22, 52))
+>CamelCase2 : Symbol(CamelCase2, Decl(genericStringMappings.ts, 10, 30))
+

tests/baselines/reference/genericStringMappings.types

@@ -0,0 +1,35 @@
+//// [tests/cases/conformance/types/literal/genericStringMappings.ts] ////
+
+=== genericStringMappings.ts ===
+// repro from #52102#issuecomment-1371248589
+
+type CamelCase1<
+>CamelCase1 : CamelCase1<S, L, Res>
+
+  S extends string,
+  L extends string = Lowercase<S>,
+  Res extends string = ""
+> = L extends ""
+  ? `start_${Uncapitalize<Res>}_end`
+  : CamelCase1<never, "", `${Res}${Capitalize<L>}`>;
+
+type Test1 = CamelCase1<"ABC">
+>Test1 : "start_abc_end"
+
+// repro from #52102
+
+type CamelCase2<
+>CamelCase2 : CamelCase2<S, L, Res>
+
+  S extends string,
+  L extends string = Lowercase<S>,
+  Res extends string = ""
+> = L extends ""
+  ? Uncapitalize<Res>
+  : L extends `${infer H}_${infer T}`
+  ? CamelCase2<never, T, `${Res}${Capitalize<H>}`>
+  : CamelCase2<never, "", `${Res}${Capitalize<L>}`>;
+
+type Test2 = CamelCase2<"FOOBAR">
+>Test2 : "foobar"
+

tests/cases/conformance/types/literal/genericStringMappings.ts

@@ -0,0 +1,28 @@
+// @strict: true
+// @noEmit: true
+
+// repro from #52102#issuecomment-1371248589
+
+type CamelCase1<
+  S extends string,
+  L extends string = Lowercase<S>,
+  Res extends string = ""
+> = L extends ""
+  ? `start_${Uncapitalize<Res>}_end`
+  : CamelCase1<never, "", `${Res}${Capitalize<L>}`>;
+
+type Test1 = CamelCase1<"ABC">
+
+// repro from #52102
+
+type CamelCase2<
+  S extends string,
+  L extends string = Lowercase<S>,
+  Res extends string = ""
+> = L extends ""
+  ? Uncapitalize<Res>
+  : L extends `${infer H}_${infer T}`
+  ? CamelCase2<never, T, `${Res}${Capitalize<H>}`>
+  : CamelCase2<never, "", `${Res}${Capitalize<L>}`>;
+
+type Test2 = CamelCase2<"FOOBAR">