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Distributive conditional type applied to 'never' produces 'never'

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This commit is contained in:
Anders Hejlsberg
2018-02-28 06:39:17 -08:00
parent daf3ed8e22
commit 56e6deefc7
1 changed files with 6 additions and 3 deletions
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9
src/compiler/checker.ts
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@@ -8229,8 +8229,8 @@ namespace ts {
const inferredTypes = map(inferences, inference => getTypeFromInference(inference) || emptyObjectType);
combinedMapper = combineTypeMappers(mapper, createTypeMapper(root.inferTypeParameters, inferredTypes));
}
// Return union of trueType and falseType for any and never since they match anything
if (checkType.flags & TypeFlags.Any || (checkType.flags & TypeFlags.Never && !(extendsType.flags & TypeFlags.Never))) {
// Return union of trueType and falseType for 'any' since it matches anything
if (checkType.flags & TypeFlags.Any) {
return getUnionType([instantiateType(root.trueType, combinedMapper || mapper), instantiateType(root.falseType, mapper)]);
}
// Instantiate the extends type including inferences for 'infer T' type parameters
@@ -8911,7 +8911,7 @@ namespace ts {
if (root.isDistributive) {
const checkType = <TypeParameter>root.checkType;
const instantiatedType = mapper(checkType);
if (checkType !== instantiatedType && instantiatedType.flags & TypeFlags.Union) {
if (checkType !== instantiatedType && instantiatedType.flags & (TypeFlags.Union | TypeFlags.Never)) {
return mapType(instantiatedType, t => getConditionalType(root, createReplacementMapper(checkType, t, mapper)));
}
}
@@ -12449,6 +12449,9 @@ namespace ts {
// is a union type, the mapping function is applied to each constituent type and a union
// of the resulting types is returned.
function mapType(type: Type, mapper: (t: Type) => Type, noReductions?: boolean): Type {
if (type.flags & TypeFlags.Never) {
return type;
}
if (!(type.flags & TypeFlags.Union)) {
return mapper(type);
}