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

Author: Andarist

src/compiler/checker.ts

@@ -12318,7 +12318,7 @@ namespace ts {
             const typeVariable = getHomomorphicTypeVariable(type);
             if (typeVariable && !type.declaration.nameType) {
                 const constraint = getConstraintOfTypeParameter(typeVariable);
-                if (constraint && isArrayOrTupleType(constraint)) {
+                if (constraint && everyType(constraint, isArrayOrTupleType)) {
                     return instantiateType(type, prependTypeMapping(typeVariable, constraint, type.mapper));
                 }
             }
@@ -19860,7 +19860,7 @@ namespace ts {
                             return varianceResult;
                         }
                     }
-                    else if (isReadonlyArrayType(target) ? isArrayOrTupleType(source) : isArrayType(target) && isTupleType(source) && !source.target.readonly) {
+                    else if (isReadonlyArrayType(target) ? everyType(source, isArrayOrTupleType) : isArrayType(target) && isTupleType(source) && !source.target.readonly) {
                         if (relation !== identityRelation) {
                             return isRelatedTo(getIndexTypeOfType(source, numberType) || anyType, getIndexTypeOfType(target, numberType) || anyType, RecursionFlags.Both, reportErrors);
                         }

tests/baselines/reference/restParamUsingMappedTypeOverUnionConstraint.symbols

@@ -0,0 +1,27 @@
+=== tests/cases/compiler/restParamUsingMappedTypeOverUnionConstraint.ts ===
+// repro 29919#issuecomment-470948453
+
+type Mapped<T> = { [P in keyof T]: T[P] extends string ? [number] : [string] }
+>Mapped : Symbol(Mapped, Decl(restParamUsingMappedTypeOverUnionConstraint.ts, 0, 0))
+>T : Symbol(T, Decl(restParamUsingMappedTypeOverUnionConstraint.ts, 2, 12))
+>P : Symbol(P, Decl(restParamUsingMappedTypeOverUnionConstraint.ts, 2, 20))
+>T : Symbol(T, Decl(restParamUsingMappedTypeOverUnionConstraint.ts, 2, 12))
+>T : Symbol(T, Decl(restParamUsingMappedTypeOverUnionConstraint.ts, 2, 12))
+>P : Symbol(P, Decl(restParamUsingMappedTypeOverUnionConstraint.ts, 2, 20))
+
+declare function test<T extends [number] | [string]>(
+>test : Symbol(test, Decl(restParamUsingMappedTypeOverUnionConstraint.ts, 2, 78))
+>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: Mapped<T>) => void
+>fn : Symbol(fn, Decl(restParamUsingMappedTypeOverUnionConstraint.ts, 5, 10))
+>args : Symbol(args, Decl(restParamUsingMappedTypeOverUnionConstraint.ts, 6, 7))
+>Mapped : Symbol(Mapped, Decl(restParamUsingMappedTypeOverUnionConstraint.ts, 0, 0))
+>T : Symbol(T, Decl(restParamUsingMappedTypeOverUnionConstraint.ts, 4, 22))
+
+): number
+

tests/baselines/reference/restParamUsingMappedTypeOverUnionConstraint.types

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

tests/cases/compiler/restParamUsingMappedTypeOverUnionConstraint.ts

@@ -0,0 +1,11 @@
+// @noEmit: true
+// @strict: true
+
+// repro 29919#issuecomment-470948453
+
+type Mapped<T> = { [P in keyof T]: T[P] extends string ? [number] : [string] }
+
+declare function test<T extends [number] | [string]>(
+  args: T,
+  fn: (...args: Mapped<T>) => void
+): number