Allow nongeneric string mapping types to exist

src/compiler/checker.ts

@@ -12145,7 +12145,7 @@ namespace ts {
                 }
                 if (t.flags & TypeFlags.StringMapping) {
                     const constraint = getBaseConstraint((t as StringMappingType).type);
-                    return constraint ? getStringMappingType((t as StringMappingType).symbol, constraint) : stringType;
+                    return constraint && constraint !== (t as StringMappingType).type ? getStringMappingType((t as StringMappingType).symbol, constraint) : stringType;
                 }
                 if (t.flags & TypeFlags.IndexedAccess) {
                     if (isMappedTypeGenericIndexedAccess(t)) {
@@ -15317,8 +15317,11 @@ namespace ts {
 
         function getStringMappingType(symbol: Symbol, type: Type): Type {
             return type.flags & (TypeFlags.Union | TypeFlags.Never) ? mapType(type, t => getStringMappingType(symbol, t)) :
-                isGenericIndexType(type) ? getStringMappingTypeForGenericType(symbol, type) :
+                // Mapping<Mapping<T>> === Mapping<T>
+                type.flags & TypeFlags.StringMapping && symbol === type.symbol ? type :
+                isGenericIndexType(type) || isPatternLiteralPlaceholderType(type) ? getStringMappingTypeForGenericType(symbol, isPatternLiteralPlaceholderType(type) && !(type.flags & TypeFlags.StringMapping) ? getTemplateLiteralType(["", ""], [type]) : type) :
                 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;
         }
 
@@ -15332,6 +15335,16 @@ namespace ts {
             return str;
         }
 
+        function applyTemplateStringMapping(symbol: Symbol, texts: readonly string[], types: readonly Type[]): [texts: readonly string[], types: readonly 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];
+            }
+            return [texts, types];
+        }
+
         function getStringMappingTypeForGenericType(symbol: Symbol, type: Type): Type {
             const id = `${getSymbolId(symbol)},${getTypeId(type)}`;
             let result = stringMappingTypes.get(id);
@@ -15587,8 +15600,8 @@ namespace ts {
                 accessNode;
         }
 
-        function isPatternLiteralPlaceholderType(type: Type) {
-            return !!(type.flags & (TypeFlags.Any | TypeFlags.String | TypeFlags.Number | TypeFlags.BigInt));
+        function isPatternLiteralPlaceholderType(type: Type): boolean {
+            return !!(type.flags & (TypeFlags.Any | TypeFlags.String | TypeFlags.Number | TypeFlags.BigInt)) || !!(type.flags & TypeFlags.StringMapping && isPatternLiteralPlaceholderType((type as StringMappingType).type));
         }
 
         function isPatternLiteralType(type: Type) {
@@ -19497,6 +19510,13 @@ namespace ts {
                         return Ternary.True;
                     }
                 }
+                else if (target.flags & TypeFlags.StringMapping) {
+                    if (!(source.flags & TypeFlags.StringMapping)) {
+                        if (isUnchangedByStringMapping(target.symbol, source)) {
+                            return Ternary.True;
+                        }
+                    }
+                }
 
                 if (sourceFlags & TypeFlags.TypeVariable) {
                     // IndexedAccess comparisons are handled above in the `targetFlags & TypeFlage.IndexedAccess` branch
@@ -19548,7 +19568,10 @@ namespace ts {
                     }
                 }
                 else if (sourceFlags & TypeFlags.StringMapping) {
-                    if (targetFlags & TypeFlags.StringMapping && (source as StringMappingType).symbol === (target as StringMappingType).symbol) {
+                    if (targetFlags & TypeFlags.StringMapping) {
+                        if ((source as StringMappingType).symbol !== (target as StringMappingType).symbol) {
+                            return Ternary.False;
+                        }
                         if (result = isRelatedTo((source as StringMappingType).type, (target as StringMappingType).type, RecursionFlags.Both, reportErrors)) {
                             resetErrorInfo(saveErrorInfo);
                             return result;
@@ -20502,7 +20525,7 @@ namespace ts {
                 }
             }
 
-            return isUnitType(type) || !!(type.flags & TypeFlags.TemplateLiteral);
+            return isUnitType(type) || !!(type.flags & TypeFlags.TemplateLiteral) || !!(type.flags & TypeFlags.StringMapping);
         }
 
         function getExactOptionalUnassignableProperties(source: Type, target: Type) {
@@ -22052,6 +22075,10 @@ namespace ts {
             return success && result === SyntaxKind.BigIntLiteral && scanner.getTextPos() === (s.length + 1) && !(flags & TokenFlags.ContainsSeparator);
         }
 
+        function isUnchangedByStringMapping(symbol: Symbol, value: Type) {
+            return value === getStringMappingType(symbol, value);
+        }
+
         function isValidTypeForTemplateLiteralPlaceholder(source: Type, target: Type): boolean {
             if (source === target || target.flags & (TypeFlags.Any | TypeFlags.String)) {
                 return true;
@@ -22060,7 +22087,8 @@ namespace ts {
                 const value = (source as StringLiteralType).value;
                 return !!(target.flags & TypeFlags.Number && value !== "" && isFinite(+value) ||
                     target.flags & TypeFlags.BigInt && value !== "" && isValidBigIntString(value) ||
-                    target.flags & (TypeFlags.BooleanLiteral | TypeFlags.Nullable) && value === (target as IntrinsicType).intrinsicName);
+                    target.flags & (TypeFlags.BooleanLiteral | TypeFlags.Nullable) && value === (target as IntrinsicType).intrinsicName ||
+                    target.flags & TypeFlags.StringMapping && isUnchangedByStringMapping(target.symbol, getStringLiteralType(value)));
             }
             if (source.flags & TypeFlags.TemplateLiteral) {
                 const texts = (source as TemplateLiteralType).texts;

tests/baselines/reference/intrinsicTypes.errors.txt

@@ -13,35 +13,35 @@ tests/cases/conformance/types/typeAliases/intrinsicTypes.ts(43,5): error TS2322:
 ==== tests/cases/conformance/types/typeAliases/intrinsicTypes.ts (8 errors) ====
     type TU1 = Uppercase<'hello'>;  // "HELLO"
     type TU2 = Uppercase<'foo' | 'bar'>;  // "FOO" | "BAR"
-    type TU3 = Uppercase<string>;  // string
-    type TU4 = Uppercase<any>;  // any
+    type TU3 = Uppercase<string>;  // Uppercase<string>
+    type TU4 = Uppercase<any>;  // Uppercase<`${any}`>
     type TU5 = Uppercase<never>;  // never
     type TU6 = Uppercase<42>;  // Error
                          ~~
 !!! error TS2344: Type 'number' does not satisfy the constraint 'string'.
 
     type TL1 = Lowercase<'HELLO'>;  // "hello"
     type TL2 = Lowercase<'FOO' | 'BAR'>;  // "foo" | "bar"
-    type TL3 = Lowercase<string>;  // string
-    type TL4 = Lowercase<any>;  // any
+    type TL3 = Lowercase<string>;  // Lowercase<string>
+    type TL4 = Lowercase<any>;  // Lowercase<`${any}`>
     type TL5 = Lowercase<never>;  // never
     type TL6 = Lowercase<42>;  // Error
                          ~~
 !!! error TS2344: Type 'number' does not satisfy the constraint 'string'.
 
     type TC1 = Capitalize<'hello'>;  // "Hello"
     type TC2 = Capitalize<'foo' | 'bar'>;  // "Foo" | "Bar"
-    type TC3 = Capitalize<string>;  // string
-    type TC4 = Capitalize<any>;  // any
+    type TC3 = Capitalize<string>;  // Capitalize<string>
+    type TC4 = Capitalize<any>;  // Capitalize<`${any}`>
     type TC5 = Capitalize<never>;  // never
     type TC6 = Capitalize<42>;  // Error
                           ~~
 !!! error TS2344: Type 'number' does not satisfy the constraint 'string'.
 
     type TN1 = Uncapitalize<'Hello'>;  // "hello"
     type TN2 = Uncapitalize<'Foo' | 'Bar'>;  // "foo" | "bar"
-    type TN3 = Uncapitalize<string>;  // string
-    type TN4 = Uncapitalize<any>;  // any
+    type TN3 = Uncapitalize<string>;  // Uncapitalize<string>
+    type TN4 = Uncapitalize<any>;  // Uncapitalize<`${any}`>
     type TN5 = Uncapitalize<never>;  // never
     type TN6 = Uncapitalize<42>;  // Error
                             ~~

tests/baselines/reference/intrinsicTypes.js

@@ -1,29 +1,29 @@
 //// [intrinsicTypes.ts]
 type TU1 = Uppercase<'hello'>;  // "HELLO"
 type TU2 = Uppercase<'foo' | 'bar'>;  // "FOO" | "BAR"
-type TU3 = Uppercase<string>;  // string
-type TU4 = Uppercase<any>;  // any
+type TU3 = Uppercase<string>;  // Uppercase<string>
+type TU4 = Uppercase<any>;  // Uppercase<`${any}`>
 type TU5 = Uppercase<never>;  // never
 type TU6 = Uppercase<42>;  // Error
 
 type TL1 = Lowercase<'HELLO'>;  // "hello"
 type TL2 = Lowercase<'FOO' | 'BAR'>;  // "foo" | "bar"
-type TL3 = Lowercase<string>;  // string
-type TL4 = Lowercase<any>;  // any
+type TL3 = Lowercase<string>;  // Lowercase<string>
+type TL4 = Lowercase<any>;  // Lowercase<`${any}`>
 type TL5 = Lowercase<never>;  // never
 type TL6 = Lowercase<42>;  // Error
 
 type TC1 = Capitalize<'hello'>;  // "Hello"
 type TC2 = Capitalize<'foo' | 'bar'>;  // "Foo" | "Bar"
-type TC3 = Capitalize<string>;  // string
-type TC4 = Capitalize<any>;  // any
+type TC3 = Capitalize<string>;  // Capitalize<string>
+type TC4 = Capitalize<any>;  // Capitalize<`${any}`>
 type TC5 = Capitalize<never>;  // never
 type TC6 = Capitalize<42>;  // Error
 
 type TN1 = Uncapitalize<'Hello'>;  // "hello"
 type TN2 = Uncapitalize<'Foo' | 'Bar'>;  // "foo" | "bar"
-type TN3 = Uncapitalize<string>;  // string
-type TN4 = Uncapitalize<any>;  // any
+type TN3 = Uncapitalize<string>;  // Uncapitalize<string>
+type TN4 = Uncapitalize<any>;  // Uncapitalize<`${any}`>
 type TN5 = Uncapitalize<never>;  // never
 type TN6 = Uncapitalize<42>;  // Error
 

tests/baselines/reference/intrinsicTypes.symbols

@@ -7,11 +7,11 @@ type TU2 = Uppercase<'foo' | 'bar'>;  // "FOO" | "BAR"
 >TU2 : Symbol(TU2, Decl(intrinsicTypes.ts, 0, 30))
 >Uppercase : Symbol(Uppercase, Decl(lib.es5.d.ts, --, --))
 
-type TU3 = Uppercase<string>;  // string
+type TU3 = Uppercase<string>;  // Uppercase<string>
 >TU3 : Symbol(TU3, Decl(intrinsicTypes.ts, 1, 36))
 >Uppercase : Symbol(Uppercase, Decl(lib.es5.d.ts, --, --))
 
-type TU4 = Uppercase<any>;  // any
+type TU4 = Uppercase<any>;  // Uppercase<`${any}`>
 >TU4 : Symbol(TU4, Decl(intrinsicTypes.ts, 2, 29))
 >Uppercase : Symbol(Uppercase, Decl(lib.es5.d.ts, --, --))
 
@@ -31,11 +31,11 @@ type TL2 = Lowercase<'FOO' | 'BAR'>;  // "foo" | "bar"
 >TL2 : Symbol(TL2, Decl(intrinsicTypes.ts, 7, 30))
 >Lowercase : Symbol(Lowercase, Decl(lib.es5.d.ts, --, --))
 
-type TL3 = Lowercase<string>;  // string
+type TL3 = Lowercase<string>;  // Lowercase<string>
 >TL3 : Symbol(TL3, Decl(intrinsicTypes.ts, 8, 36))
 >Lowercase : Symbol(Lowercase, Decl(lib.es5.d.ts, --, --))
 
-type TL4 = Lowercase<any>;  // any
+type TL4 = Lowercase<any>;  // Lowercase<`${any}`>
 >TL4 : Symbol(TL4, Decl(intrinsicTypes.ts, 9, 29))
 >Lowercase : Symbol(Lowercase, Decl(lib.es5.d.ts, --, --))
 
@@ -55,11 +55,11 @@ type TC2 = Capitalize<'foo' | 'bar'>;  // "Foo" | "Bar"
 >TC2 : Symbol(TC2, Decl(intrinsicTypes.ts, 14, 31))
 >Capitalize : Symbol(Capitalize, Decl(lib.es5.d.ts, --, --))
 
-type TC3 = Capitalize<string>;  // string
+type TC3 = Capitalize<string>;  // Capitalize<string>
 >TC3 : Symbol(TC3, Decl(intrinsicTypes.ts, 15, 37))
 >Capitalize : Symbol(Capitalize, Decl(lib.es5.d.ts, --, --))
 
-type TC4 = Capitalize<any>;  // any
+type TC4 = Capitalize<any>;  // Capitalize<`${any}`>
 >TC4 : Symbol(TC4, Decl(intrinsicTypes.ts, 16, 30))
 >Capitalize : Symbol(Capitalize, Decl(lib.es5.d.ts, --, --))
 
@@ -79,11 +79,11 @@ type TN2 = Uncapitalize<'Foo' | 'Bar'>;  // "foo" | "bar"
 >TN2 : Symbol(TN2, Decl(intrinsicTypes.ts, 21, 33))
 >Uncapitalize : Symbol(Uncapitalize, Decl(lib.es5.d.ts, --, --))
 
-type TN3 = Uncapitalize<string>;  // string
+type TN3 = Uncapitalize<string>;  // Uncapitalize<string>
 >TN3 : Symbol(TN3, Decl(intrinsicTypes.ts, 22, 39))
 >Uncapitalize : Symbol(Uncapitalize, Decl(lib.es5.d.ts, --, --))
 
-type TN4 = Uncapitalize<any>;  // any
+type TN4 = Uncapitalize<any>;  // Uncapitalize<`${any}`>
 >TN4 : Symbol(TN4, Decl(intrinsicTypes.ts, 23, 32))
 >Uncapitalize : Symbol(Uncapitalize, Decl(lib.es5.d.ts, --, --))
 

tests/baselines/reference/intrinsicTypes.types

@@ -5,11 +5,11 @@ type TU1 = Uppercase<'hello'>;  // "HELLO"
 type TU2 = Uppercase<'foo' | 'bar'>;  // "FOO" | "BAR"
 >TU2 : "FOO" | "BAR"
 
-type TU3 = Uppercase<string>;  // string
->TU3 : string
+type TU3 = Uppercase<string>;  // Uppercase<string>
+>TU3 : Uppercase<string>
 
-type TU4 = Uppercase<any>;  // any
->TU4 : any
+type TU4 = Uppercase<any>;  // Uppercase<`${any}`>
+>TU4 : Uppercase<`${any}`>
 
 type TU5 = Uppercase<never>;  // never
 >TU5 : never
@@ -23,11 +23,11 @@ type TL1 = Lowercase<'HELLO'>;  // "hello"
 type TL2 = Lowercase<'FOO' | 'BAR'>;  // "foo" | "bar"
 >TL2 : "foo" | "bar"
 
-type TL3 = Lowercase<string>;  // string
->TL3 : string
+type TL3 = Lowercase<string>;  // Lowercase<string>
+>TL3 : Lowercase<string>
 
-type TL4 = Lowercase<any>;  // any
->TL4 : any
+type TL4 = Lowercase<any>;  // Lowercase<`${any}`>
+>TL4 : Lowercase<`${any}`>
 
 type TL5 = Lowercase<never>;  // never
 >TL5 : never
@@ -41,11 +41,11 @@ type TC1 = Capitalize<'hello'>;  // "Hello"
 type TC2 = Capitalize<'foo' | 'bar'>;  // "Foo" | "Bar"
 >TC2 : "Foo" | "Bar"
 
-type TC3 = Capitalize<string>;  // string
->TC3 : string
+type TC3 = Capitalize<string>;  // Capitalize<string>
+>TC3 : Capitalize<string>
 
-type TC4 = Capitalize<any>;  // any
->TC4 : any
+type TC4 = Capitalize<any>;  // Capitalize<`${any}`>
+>TC4 : Capitalize<`${any}`>
 
 type TC5 = Capitalize<never>;  // never
 >TC5 : never
@@ -59,11 +59,11 @@ type TN1 = Uncapitalize<'Hello'>;  // "hello"
 type TN2 = Uncapitalize<'Foo' | 'Bar'>;  // "foo" | "bar"
 >TN2 : "foo" | "bar"
 
-type TN3 = Uncapitalize<string>;  // string
->TN3 : string
+type TN3 = Uncapitalize<string>;  // Uncapitalize<string>
+>TN3 : Uncapitalize<string>
 
-type TN4 = Uncapitalize<any>;  // any
->TN4 : any
+type TN4 = Uncapitalize<any>;  // Uncapitalize<`${any}`>
+>TN4 : Uncapitalize<`${any}`>
 
 type TN5 = Uncapitalize<never>;  // never
 >TN5 : never

tests/baselines/reference/mappedTypeConstraints2.errors.txt

@@ -1,7 +1,8 @@
 tests/cases/conformance/types/mapped/mappedTypeConstraints2.ts(10,11): error TS2322: Type 'Mapped2<K>[`get${K}`]' is not assignable to type '{ a: K; }'.
   Type 'Mapped2<K>[`get${string}`]' is not assignable to type '{ a: K; }'.
 tests/cases/conformance/types/mapped/mappedTypeConstraints2.ts(16,11): error TS2322: Type 'Mapped3<K>[Uppercase<K>]' is not assignable to type '{ a: K; }'.
-  Type 'Mapped3<K>[string]' is not assignable to type '{ a: K; }'.
+  Type 'Mapped3<K>[Uppercase<string>]' is not assignable to type '{ a: K; }'.
+    Type 'Mapped3<K>[string]' is not assignable to type '{ a: K; }'.
 tests/cases/conformance/types/mapped/mappedTypeConstraints2.ts(25,57): error TS2322: Type 'Foo<T>[`get${T}`]' is not assignable to type 'T'.
   'T' could be instantiated with an arbitrary type which could be unrelated to 'Foo<T>[`get${T}`]'.
 
@@ -28,7 +29,8 @@ tests/cases/conformance/types/mapped/mappedTypeConstraints2.ts(25,57): error TS2
         const x: { a: K } = obj[key];  // Error
               ~
 !!! error TS2322: Type 'Mapped3<K>[Uppercase<K>]' is not assignable to type '{ a: K; }'.
-!!! error TS2322:   Type 'Mapped3<K>[string]' is not assignable to type '{ a: K; }'.
+!!! error TS2322:   Type 'Mapped3<K>[Uppercase<string>]' is not assignable to type '{ a: K; }'.
+!!! error TS2322:     Type 'Mapped3<K>[string]' is not assignable to type '{ a: K; }'.
     }
 
     // Repro from #47794