Bring back resolving apparent type for mapped types with tuple constr…

…aints but avoid resolving the apparent type of mapped types for contextual types
microsoft · Mar 19, 2023 · 940e1ad · 940e1ad
1 parent 29d4bb9
commit 940e1ad
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Showing 3 changed files with 10 additions and 4 deletions.
diff --git a/src/compiler/checker.ts b/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 && everyType(constraint, isArrayType)) {
+            if (constraint && everyType(constraint, isArrayOrTupleType)) {
                 return instantiateType(type, prependTypeMapping(typeVariable, constraint, type.mapper));
             }
         }
@@ -29205,7 +29205,15 @@ 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;

diff --git a/tests/baselines/reference/mappedTypeUnionConstrainTupleTreatedAsArrayLike.types b/tests/baselines/reference/mappedTypeUnionConstrainTupleTreatedAsArrayLike.types
@@ -1,7 +1,6 @@
 === tests/cases/compiler/mappedTypeUnionConstrainTupleTreatedAsArrayLike.ts ===
 type HomomorphicMappedType<T> = { [P in keyof T]: T[P] extends string ? boolean : null }
 >HomomorphicMappedType : HomomorphicMappedType<T>
->null : null
 
 function test1<T extends [number] | [string]>(args: T) {
 >test1 : <T extends [number] | [string]>(args: T) => void

diff --git a/tests/baselines/reference/restParamUsingMappedTypeOverUnionConstraint.types b/tests/baselines/reference/restParamUsingMappedTypeOverUnionConstraint.types
@@ -3,7 +3,6 @@
 
 type HomomorphicMappedType<T> = { [P in keyof T]: T[P] extends string ? boolean : null }
 >HomomorphicMappedType : HomomorphicMappedType<T>
->null : null
 
 declare function test<T extends [number] | [string]>(
 >test : <T extends [number] | [string]>(args: T, fn: (...args: HomomorphicMappedType<T>) => void) => void