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Fix subtype reduction involving type variables with union constraints (#53351)

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This commit is contained in:
Anders Hejlsberg
2023-03-21 12:52:05 -07:00
committed by GitHub
parent abb4052f2f
commit bace6897e2
4 changed files with 261 additions and 0 deletions
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10
src/compiler/checker.ts
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@@ -16276,6 +16276,16 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
i--;
const source = types[i];
if (hasEmptyObject || source.flags & TypeFlags.StructuredOrInstantiable) {
// A type parameter with a union constraint may be a subtype of some union, but not a subtype of the
// individual constituents of that union. For example, `T extends A | B` is a subtype of `A | B`, but not
// a subtype of just `A` or just `B`. When we encounter such a type parameter, we therefore check if the
// type parameter is a subtype of a union of all the other types.
if (source.flags & TypeFlags.TypeParameter && getBaseConstraintOrType(source).flags & TypeFlags.Union) {
if (isTypeRelatedTo(source, getUnionType(map(types, t => t === source ? neverType : t)), strictSubtypeRelation)) {
orderedRemoveItemAt(types, i);
}
continue;
}
// Find the first property with a unit type, if any. When constituents have a property by the same name
// but of a different unit type, we can quickly disqualify them from subtype checks. This helps subtype
// reduction of large discriminated union types.