Fixed an issue with not being able to use mapped type over union constraint as rest param

src/compiler/checker.ts

@@ -13861,7 +13861,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
         const typeVariable = getHomomorphicTypeVariable(type);
         if (typeVariable && !type.declaration.nameType) {
             const constraint = getConstraintOfTypeParameter(typeVariable);
-            if (constraint && isArrayType(constraint)) {
+            if (constraint && everyType(constraint, isArrayOrTupleType)) {
                 return instantiateType(type, prependTypeMapping(typeVariable, constraint, type.mapper));
             }
         }
@@ -21679,7 +21679,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
                         return varianceResult;
                     }
                 }
-                else if (isReadonlyArrayType(target) ? isArrayOrTupleType(source) : isArrayType(target) && isTupleType(source) && !source.target.readonly) {
+                else if (isReadonlyArrayType(target) ? everyType(source, isArrayOrTupleType) : isArrayType(target) && everyType(source, t => isTupleType(t) && !t.target.readonly)) {
                     if (relation !== identityRelation) {
                         return isRelatedTo(getIndexTypeOfType(source, numberType) || anyType, getIndexTypeOfType(target, numberType) || anyType, RecursionFlags.Both, reportErrors);
                     }
@@ -29251,7 +29251,15 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
             getContextualType(node, contextFlags);
         const instantiatedType = instantiateContextualType(contextualType, node, contextFlags);
         if (instantiatedType && !(contextFlags && contextFlags & ContextFlags.NoConstraints && instantiatedType.flags & TypeFlags.TypeVariable)) {
-            const apparentType = mapType(instantiatedType, getApparentType, /*noReductions*/ true);
+            const apparentType = mapType(
+                instantiatedType,
+                // When obtaining apparent type of *contextual* type we don't want to get apparent type of mapped types.
+                // That would evaluate mapped types with array or tuple type constraints too eagerly
+                // and thus it would prevent `getTypeOfPropertyOfContextualType` from obtaining per-position contextual type for elements of array literal expressions.
+                // Apparent type of other mapped types is already the mapped type itself so we can just avoid calling `getApparentType` here for all mapped types.
+                t => getObjectFlags(t) & ObjectFlags.Mapped ? t : getApparentType(t),
+                /*noReductions*/ true
+            );
             return apparentType.flags & TypeFlags.Union && isObjectLiteralExpression(node) ? discriminateContextualTypeByObjectMembers(node, apparentType as UnionType) :
                 apparentType.flags & TypeFlags.Union && isJsxAttributes(node) ? discriminateContextualTypeByJSXAttributes(node, apparentType as UnionType) :
                 apparentType;

tests/baselines/reference/mappedTypeTupleConstraintAssignability.symbols

@@ -0,0 +1,134 @@
+=== tests/cases/compiler/mappedTypeTupleConstraintAssignability.ts ===
+// https://github.com/microsoft/TypeScript/issues/53359#issuecomment-1475390594
+
+type Writeable<T> = { -readonly [P in keyof T]: T[P] };
+>Writeable : Symbol(Writeable, Decl(mappedTypeTupleConstraintAssignability.ts, 0, 0))
+>T : Symbol(T, Decl(mappedTypeTupleConstraintAssignability.ts, 2, 15))
+>P : Symbol(P, Decl(mappedTypeTupleConstraintAssignability.ts, 2, 33))
+>T : Symbol(T, Decl(mappedTypeTupleConstraintAssignability.ts, 2, 15))
+>T : Symbol(T, Decl(mappedTypeTupleConstraintAssignability.ts, 2, 15))
+>P : Symbol(P, Decl(mappedTypeTupleConstraintAssignability.ts, 2, 33))
+
+type EnumValues = [string, ...string[]];
+>EnumValues : Symbol(EnumValues, Decl(mappedTypeTupleConstraintAssignability.ts, 2, 55))
+
+type Values<T extends EnumValues> = { [k in T[number]]: k; };
+>Values : Symbol(Values, Decl(mappedTypeTupleConstraintAssignability.ts, 3, 40))
+>T : Symbol(T, Decl(mappedTypeTupleConstraintAssignability.ts, 4, 12))
+>EnumValues : Symbol(EnumValues, Decl(mappedTypeTupleConstraintAssignability.ts, 2, 55))
+>k : Symbol(k, Decl(mappedTypeTupleConstraintAssignability.ts, 4, 39))
+>T : Symbol(T, Decl(mappedTypeTupleConstraintAssignability.ts, 4, 12))
+>k : Symbol(k, Decl(mappedTypeTupleConstraintAssignability.ts, 4, 39))
+
+declare class ZodEnum<T extends [string, ...string[]]> {
+>ZodEnum : Symbol(ZodEnum, Decl(mappedTypeTupleConstraintAssignability.ts, 4, 61))
+>T : Symbol(T, Decl(mappedTypeTupleConstraintAssignability.ts, 6, 22))
+
+  get enum(): Values<T>
+>enum : Symbol(ZodEnum.enum, Decl(mappedTypeTupleConstraintAssignability.ts, 6, 56))
+>Values : Symbol(Values, Decl(mappedTypeTupleConstraintAssignability.ts, 3, 40))
+>T : Symbol(T, Decl(mappedTypeTupleConstraintAssignability.ts, 6, 22))
+}
+
+declare function createZodEnum<U extends string, T extends Readonly<[U, ...U[]]>>(values: T): ZodEnum<Writeable<T>>;
+>createZodEnum : Symbol(createZodEnum, Decl(mappedTypeTupleConstraintAssignability.ts, 8, 1))
+>U : Symbol(U, Decl(mappedTypeTupleConstraintAssignability.ts, 10, 31))
+>T : Symbol(T, Decl(mappedTypeTupleConstraintAssignability.ts, 10, 48))
+>Readonly : Symbol(Readonly, Decl(lib.es5.d.ts, --, --))
+>U : Symbol(U, Decl(mappedTypeTupleConstraintAssignability.ts, 10, 31))
+>U : Symbol(U, Decl(mappedTypeTupleConstraintAssignability.ts, 10, 31))
+>values : Symbol(values, Decl(mappedTypeTupleConstraintAssignability.ts, 10, 82))
+>T : Symbol(T, Decl(mappedTypeTupleConstraintAssignability.ts, 10, 48))
+>ZodEnum : Symbol(ZodEnum, Decl(mappedTypeTupleConstraintAssignability.ts, 4, 61))
+>Writeable : Symbol(Writeable, Decl(mappedTypeTupleConstraintAssignability.ts, 0, 0))
+>T : Symbol(T, Decl(mappedTypeTupleConstraintAssignability.ts, 10, 48))
+
+// https://github.com/microsoft/TypeScript/issues/53359#issuecomment-1475390607
+
+type Maybe<T> = T | null | undefined;
+>Maybe : Symbol(Maybe, Decl(mappedTypeTupleConstraintAssignability.ts, 10, 116))
+>T : Symbol(T, Decl(mappedTypeTupleConstraintAssignability.ts, 14, 11))
+>T : Symbol(T, Decl(mappedTypeTupleConstraintAssignability.ts, 14, 11))
+
+type AnyTuple = [unknown, ...unknown[]];
+>AnyTuple : Symbol(AnyTuple, Decl(mappedTypeTupleConstraintAssignability.ts, 14, 37))
+
+type AnyObject = { [k: string]: any };
+>AnyObject : Symbol(AnyObject, Decl(mappedTypeTupleConstraintAssignability.ts, 15, 40))
+>k : Symbol(k, Decl(mappedTypeTupleConstraintAssignability.ts, 16, 20))
+
+type Flags = "s" | "d" | "";
+>Flags : Symbol(Flags, Decl(mappedTypeTupleConstraintAssignability.ts, 16, 38))
+
+interface ISchema<T, C = any, F extends Flags = any, D = any> {
+>ISchema : Symbol(ISchema, Decl(mappedTypeTupleConstraintAssignability.ts, 17, 28))
+>T : Symbol(T, Decl(mappedTypeTupleConstraintAssignability.ts, 19, 18))
+>C : Symbol(C, Decl(mappedTypeTupleConstraintAssignability.ts, 19, 20))
+>F : Symbol(F, Decl(mappedTypeTupleConstraintAssignability.ts, 19, 29))
+>Flags : Symbol(Flags, Decl(mappedTypeTupleConstraintAssignability.ts, 16, 38))
+>D : Symbol(D, Decl(mappedTypeTupleConstraintAssignability.ts, 19, 52))
+
+  __flags: F;
+>__flags : Symbol(ISchema.__flags, Decl(mappedTypeTupleConstraintAssignability.ts, 19, 63))
+>F : Symbol(F, Decl(mappedTypeTupleConstraintAssignability.ts, 19, 29))
+
+  __context: C;
+>__context : Symbol(ISchema.__context, Decl(mappedTypeTupleConstraintAssignability.ts, 20, 13))
+>C : Symbol(C, Decl(mappedTypeTupleConstraintAssignability.ts, 19, 20))
+
+  __outputType: T;
+>__outputType : Symbol(ISchema.__outputType, Decl(mappedTypeTupleConstraintAssignability.ts, 21, 15))
+>T : Symbol(T, Decl(mappedTypeTupleConstraintAssignability.ts, 19, 18))
+
+  __default: D;
+>__default : Symbol(ISchema.__default, Decl(mappedTypeTupleConstraintAssignability.ts, 22, 18))
+>D : Symbol(D, Decl(mappedTypeTupleConstraintAssignability.ts, 19, 52))
+}
+
+declare class TupleSchema<
+>TupleSchema : Symbol(TupleSchema, Decl(mappedTypeTupleConstraintAssignability.ts, 24, 1))
+
+  TType extends Maybe<AnyTuple> = AnyTuple | undefined,
+>TType : Symbol(TType, Decl(mappedTypeTupleConstraintAssignability.ts, 26, 26))
+>Maybe : Symbol(Maybe, Decl(mappedTypeTupleConstraintAssignability.ts, 10, 116))
+>AnyTuple : Symbol(AnyTuple, Decl(mappedTypeTupleConstraintAssignability.ts, 14, 37))
+>AnyTuple : Symbol(AnyTuple, Decl(mappedTypeTupleConstraintAssignability.ts, 14, 37))
+
+  TContext = AnyObject,
+>TContext : Symbol(TContext, Decl(mappedTypeTupleConstraintAssignability.ts, 27, 55))
+>AnyObject : Symbol(AnyObject, Decl(mappedTypeTupleConstraintAssignability.ts, 15, 40))
+
+  TDefault = undefined,
+>TDefault : Symbol(TDefault, Decl(mappedTypeTupleConstraintAssignability.ts, 28, 23))
+
+  TFlags extends Flags = ""
+>TFlags : Symbol(TFlags, Decl(mappedTypeTupleConstraintAssignability.ts, 29, 23))
+>Flags : Symbol(Flags, Decl(mappedTypeTupleConstraintAssignability.ts, 16, 38))
+
+> {
+  constructor(schemas: [ISchema<any>, ...ISchema<any>[]]);
+>schemas : Symbol(schemas, Decl(mappedTypeTupleConstraintAssignability.ts, 32, 14))
+>ISchema : Symbol(ISchema, Decl(mappedTypeTupleConstraintAssignability.ts, 17, 28))
+>ISchema : Symbol(ISchema, Decl(mappedTypeTupleConstraintAssignability.ts, 17, 28))
+}
+
+export function create<T extends AnyTuple>(schemas: {
+>create : Symbol(create, Decl(mappedTypeTupleConstraintAssignability.ts, 33, 1))
+>T : Symbol(T, Decl(mappedTypeTupleConstraintAssignability.ts, 35, 23))
+>AnyTuple : Symbol(AnyTuple, Decl(mappedTypeTupleConstraintAssignability.ts, 14, 37))
+>schemas : Symbol(schemas, Decl(mappedTypeTupleConstraintAssignability.ts, 35, 43))
+
+  [K in keyof T]: ISchema<T[K]>;
+>K : Symbol(K, Decl(mappedTypeTupleConstraintAssignability.ts, 36, 3))
+>T : Symbol(T, Decl(mappedTypeTupleConstraintAssignability.ts, 35, 23))
+>ISchema : Symbol(ISchema, Decl(mappedTypeTupleConstraintAssignability.ts, 17, 28))
+>T : Symbol(T, Decl(mappedTypeTupleConstraintAssignability.ts, 35, 23))
+>K : Symbol(K, Decl(mappedTypeTupleConstraintAssignability.ts, 36, 3))
+
+}) {
+  return new TupleSchema<T | undefined>(schemas);
+>TupleSchema : Symbol(TupleSchema, Decl(mappedTypeTupleConstraintAssignability.ts, 24, 1))
+>T : Symbol(T, Decl(mappedTypeTupleConstraintAssignability.ts, 35, 23))
+>schemas : Symbol(schemas, Decl(mappedTypeTupleConstraintAssignability.ts, 35, 43))
+}
+

tests/baselines/reference/mappedTypeTupleConstraintAssignability.types

@@ -0,0 +1,76 @@
+=== tests/cases/compiler/mappedTypeTupleConstraintAssignability.ts ===
+// https://github.com/microsoft/TypeScript/issues/53359#issuecomment-1475390594
+
+type Writeable<T> = { -readonly [P in keyof T]: T[P] };
+>Writeable : Writeable<T>
+
+type EnumValues = [string, ...string[]];
+>EnumValues : [string, ...string[]]
+
+type Values<T extends EnumValues> = { [k in T[number]]: k; };
+>Values : Values<T>
+
+declare class ZodEnum<T extends [string, ...string[]]> {
+>ZodEnum : ZodEnum<T>
+
+  get enum(): Values<T>
+>enum : Values<T>
+}
+
+declare function createZodEnum<U extends string, T extends Readonly<[U, ...U[]]>>(values: T): ZodEnum<Writeable<T>>;
+>createZodEnum : <U extends string, T extends readonly [U, ...U[]]>(values: T) => ZodEnum<Writeable<T>>
+>values : T
+
+// https://github.com/microsoft/TypeScript/issues/53359#issuecomment-1475390607
+
+type Maybe<T> = T | null | undefined;
+>Maybe : Maybe<T>
+
+type AnyTuple = [unknown, ...unknown[]];
+>AnyTuple : [unknown, ...unknown[]]
+
+type AnyObject = { [k: string]: any };
+>AnyObject : { [k: string]: any; }
+>k : string
+
+type Flags = "s" | "d" | "";
+>Flags : "" | "s" | "d"
+
+interface ISchema<T, C = any, F extends Flags = any, D = any> {
+  __flags: F;
+>__flags : F
+
+  __context: C;
+>__context : C
+
+  __outputType: T;
+>__outputType : T
+
+  __default: D;
+>__default : D
+}
+
+declare class TupleSchema<
+>TupleSchema : TupleSchema<TType, TContext, TDefault, TFlags>
+
+  TType extends Maybe<AnyTuple> = AnyTuple | undefined,
+  TContext = AnyObject,
+  TDefault = undefined,
+  TFlags extends Flags = ""
+> {
+  constructor(schemas: [ISchema<any>, ...ISchema<any>[]]);
+>schemas : [ISchema<any, any, any, any>, ...ISchema<any, any, any, any>[]]
+}
+
+export function create<T extends AnyTuple>(schemas: {
+>create : <T extends AnyTuple>(schemas: { [K in keyof T]: ISchema<T[K], any, any, any>; }) => TupleSchema<T | undefined, AnyObject, undefined, "">
+>schemas : { [K in keyof T]: ISchema<T[K], any, any, any>; }
+
+  [K in keyof T]: ISchema<T[K]>;
+}) {
+  return new TupleSchema<T | undefined>(schemas);
+>new TupleSchema<T | undefined>(schemas) : TupleSchema<T | undefined, AnyObject, undefined, "">
+>TupleSchema : typeof TupleSchema
+>schemas : { [K in keyof T]: ISchema<T[K], any, any, any>; }
+}
+

tests/baselines/reference/mappedTypeUnionConstrainTupleTreatedAsArrayLike.errors.txt

@@ -0,0 +1,18 @@
+tests/cases/compiler/mappedTypeUnionConstrainTupleTreatedAsArrayLike.ts(9,9): error TS2322: Type 'HomomorphicMappedType<T>' is not assignable to type 'any[]'.
+
+
+==== tests/cases/compiler/mappedTypeUnionConstrainTupleTreatedAsArrayLike.ts (1 errors) ====
+    type HomomorphicMappedType<T> = { [P in keyof T]: T[P] extends string ? boolean : null }
+
+    function test1<T extends [number] | [string]>(args: T) {
+      const arr: any[] = [] as HomomorphicMappedType<T>
+      const arr2: readonly any[] = [] as HomomorphicMappedType<T>
+    }
+
+    function test2<T extends [number] | readonly [string]>(args: T) {
+      const arr: any[] = [] as HomomorphicMappedType<T> // error
+            ~~~
+!!! error TS2322: Type 'HomomorphicMappedType<T>' is not assignable to type 'any[]'.
+      const arr2: readonly any[] = [] as HomomorphicMappedType<T>
+    }
+

tests/baselines/reference/mappedTypeUnionConstrainTupleTreatedAsArrayLike.symbols

@@ -0,0 +1,43 @@
+=== tests/cases/compiler/mappedTypeUnionConstrainTupleTreatedAsArrayLike.ts ===
+type HomomorphicMappedType<T> = { [P in keyof T]: T[P] extends string ? boolean : null }
+>HomomorphicMappedType : Symbol(HomomorphicMappedType, Decl(mappedTypeUnionConstrainTupleTreatedAsArrayLike.ts, 0, 0))
+>T : Symbol(T, Decl(mappedTypeUnionConstrainTupleTreatedAsArrayLike.ts, 0, 27))
+>P : Symbol(P, Decl(mappedTypeUnionConstrainTupleTreatedAsArrayLike.ts, 0, 35))
+>T : Symbol(T, Decl(mappedTypeUnionConstrainTupleTreatedAsArrayLike.ts, 0, 27))
+>T : Symbol(T, Decl(mappedTypeUnionConstrainTupleTreatedAsArrayLike.ts, 0, 27))
+>P : Symbol(P, Decl(mappedTypeUnionConstrainTupleTreatedAsArrayLike.ts, 0, 35))
+
+function test1<T extends [number] | [string]>(args: T) {
+>test1 : Symbol(test1, Decl(mappedTypeUnionConstrainTupleTreatedAsArrayLike.ts, 0, 88))
+>T : Symbol(T, Decl(mappedTypeUnionConstrainTupleTreatedAsArrayLike.ts, 2, 15))
+>args : Symbol(args, Decl(mappedTypeUnionConstrainTupleTreatedAsArrayLike.ts, 2, 46))
+>T : Symbol(T, Decl(mappedTypeUnionConstrainTupleTreatedAsArrayLike.ts, 2, 15))
+
+  const arr: any[] = [] as HomomorphicMappedType<T>
+>arr : Symbol(arr, Decl(mappedTypeUnionConstrainTupleTreatedAsArrayLike.ts, 3, 7))
+>HomomorphicMappedType : Symbol(HomomorphicMappedType, Decl(mappedTypeUnionConstrainTupleTreatedAsArrayLike.ts, 0, 0))
+>T : Symbol(T, Decl(mappedTypeUnionConstrainTupleTreatedAsArrayLike.ts, 2, 15))
+
+  const arr2: readonly any[] = [] as HomomorphicMappedType<T>
+>arr2 : Symbol(arr2, Decl(mappedTypeUnionConstrainTupleTreatedAsArrayLike.ts, 4, 7))
+>HomomorphicMappedType : Symbol(HomomorphicMappedType, Decl(mappedTypeUnionConstrainTupleTreatedAsArrayLike.ts, 0, 0))
+>T : Symbol(T, Decl(mappedTypeUnionConstrainTupleTreatedAsArrayLike.ts, 2, 15))
+}
+
+function test2<T extends [number] | readonly [string]>(args: T) {
+>test2 : Symbol(test2, Decl(mappedTypeUnionConstrainTupleTreatedAsArrayLike.ts, 5, 1))
+>T : Symbol(T, Decl(mappedTypeUnionConstrainTupleTreatedAsArrayLike.ts, 7, 15))
+>args : Symbol(args, Decl(mappedTypeUnionConstrainTupleTreatedAsArrayLike.ts, 7, 55))
+>T : Symbol(T, Decl(mappedTypeUnionConstrainTupleTreatedAsArrayLike.ts, 7, 15))
+
+  const arr: any[] = [] as HomomorphicMappedType<T> // error
+>arr : Symbol(arr, Decl(mappedTypeUnionConstrainTupleTreatedAsArrayLike.ts, 8, 7))
+>HomomorphicMappedType : Symbol(HomomorphicMappedType, Decl(mappedTypeUnionConstrainTupleTreatedAsArrayLike.ts, 0, 0))
+>T : Symbol(T, Decl(mappedTypeUnionConstrainTupleTreatedAsArrayLike.ts, 7, 15))
+
+  const arr2: readonly any[] = [] as HomomorphicMappedType<T>
+>arr2 : Symbol(arr2, Decl(mappedTypeUnionConstrainTupleTreatedAsArrayLike.ts, 9, 7))
+>HomomorphicMappedType : Symbol(HomomorphicMappedType, Decl(mappedTypeUnionConstrainTupleTreatedAsArrayLike.ts, 0, 0))
+>T : Symbol(T, Decl(mappedTypeUnionConstrainTupleTreatedAsArrayLike.ts, 7, 15))
+}
+

tests/baselines/reference/mappedTypeUnionConstrainTupleTreatedAsArrayLike.types

@@ -0,0 +1,34 @@
+=== tests/cases/compiler/mappedTypeUnionConstrainTupleTreatedAsArrayLike.ts ===
+type HomomorphicMappedType<T> = { [P in keyof T]: T[P] extends string ? boolean : null }
+>HomomorphicMappedType : HomomorphicMappedType<T>
+
+function test1<T extends [number] | [string]>(args: T) {
+>test1 : <T extends [number] | [string]>(args: T) => void
+>args : T
+
+  const arr: any[] = [] as HomomorphicMappedType<T>
+>arr : any[]
+>[] as HomomorphicMappedType<T> : HomomorphicMappedType<T>
+>[] : []
+
+  const arr2: readonly any[] = [] as HomomorphicMappedType<T>
+>arr2 : readonly any[]
+>[] as HomomorphicMappedType<T> : HomomorphicMappedType<T>
+>[] : []
+}
+
+function test2<T extends [number] | readonly [string]>(args: T) {
+>test2 : <T extends [number] | readonly [string]>(args: T) => void
+>args : T
+
+  const arr: any[] = [] as HomomorphicMappedType<T> // error
+>arr : any[]
+>[] as HomomorphicMappedType<T> : HomomorphicMappedType<T>
+>[] : []
+
+  const arr2: readonly any[] = [] as HomomorphicMappedType<T>
+>arr2 : readonly any[]
+>[] as HomomorphicMappedType<T> : HomomorphicMappedType<T>
+>[] : []
+}
+

tests/baselines/reference/restParamUsingMappedTypeOverUnionConstraint.symbols

@@ -0,0 +1,27 @@
+=== tests/cases/compiler/restParamUsingMappedTypeOverUnionConstraint.ts ===
+// repro 29919#issuecomment-470948453
+
+type HomomorphicMappedType<T> = { [P in keyof T]: T[P] extends string ? boolean : null }
+>HomomorphicMappedType : Symbol(HomomorphicMappedType, Decl(restParamUsingMappedTypeOverUnionConstraint.ts, 0, 0))
+>T : Symbol(T, Decl(restParamUsingMappedTypeOverUnionConstraint.ts, 2, 27))
+>P : Symbol(P, Decl(restParamUsingMappedTypeOverUnionConstraint.ts, 2, 35))
+>T : Symbol(T, Decl(restParamUsingMappedTypeOverUnionConstraint.ts, 2, 27))
+>T : Symbol(T, Decl(restParamUsingMappedTypeOverUnionConstraint.ts, 2, 27))
+>P : Symbol(P, Decl(restParamUsingMappedTypeOverUnionConstraint.ts, 2, 35))
+
+declare function test<T extends [number] | [string]>(
+>test : Symbol(test, Decl(restParamUsingMappedTypeOverUnionConstraint.ts, 2, 88))
+>T : Symbol(T, Decl(restParamUsingMappedTypeOverUnionConstraint.ts, 4, 22))
+
+  args: T,
+>args : Symbol(args, Decl(restParamUsingMappedTypeOverUnionConstraint.ts, 4, 53))
+>T : Symbol(T, Decl(restParamUsingMappedTypeOverUnionConstraint.ts, 4, 22))
+
+  fn: (...args: HomomorphicMappedType<T>) => void
+>fn : Symbol(fn, Decl(restParamUsingMappedTypeOverUnionConstraint.ts, 5, 10))
+>args : Symbol(args, Decl(restParamUsingMappedTypeOverUnionConstraint.ts, 6, 7))
+>HomomorphicMappedType : Symbol(HomomorphicMappedType, Decl(restParamUsingMappedTypeOverUnionConstraint.ts, 0, 0))
+>T : Symbol(T, Decl(restParamUsingMappedTypeOverUnionConstraint.ts, 4, 22))
+
+): void
+

tests/baselines/reference/restParamUsingMappedTypeOverUnionConstraint.types

@@ -0,0 +1,18 @@
+=== tests/cases/compiler/restParamUsingMappedTypeOverUnionConstraint.ts ===
+// repro 29919#issuecomment-470948453
+
+type HomomorphicMappedType<T> = { [P in keyof T]: T[P] extends string ? boolean : null }
+>HomomorphicMappedType : HomomorphicMappedType<T>
+
+declare function test<T extends [number] | [string]>(
+>test : <T extends [number] | [string]>(args: T, fn: (...args: HomomorphicMappedType<T>) => void) => void
+
+  args: T,
+>args : T
+
+  fn: (...args: HomomorphicMappedType<T>) => void
+>fn : (...args: HomomorphicMappedType<T>) => void
+>args : HomomorphicMappedType<T>
+
+): void
+