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Only conditional types that check naked type parameter distribute over unions

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This commit is contained in:
Anders Hejlsberg
2018-01-24 13:50:31 -08:00
parent 4c7ec3c51a
commit b42c6b1db6
1 changed files with 3 additions and 7 deletions
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10
src/compiler/checker.ts
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@@ -8092,10 +8092,6 @@ namespace ts {
}
function getConditionalType(checkType: Type, extendsType: Type, baseTrueType: Type, baseFalseType: Type, target: ConditionalType, mapper: TypeMapper, aliasSymbol?: Symbol, baseAliasTypeArguments?: Type[]): Type {
// Distribute union types over conditional types
if (checkType.flags & TypeFlags.Union) {
return getUnionType(map((<UnionType>checkType).types, t => getConditionalType(t, extendsType, baseTrueType, baseFalseType, target, mapper)));
}
// Return union of trueType and falseType for any and never since they match anything
if (checkType.flags & (TypeFlags.Any | TypeFlags.Never)) {
return getUnionType([instantiateType(baseTrueType, mapper), instantiateType(baseFalseType, mapper)]);
@@ -8105,7 +8101,7 @@ namespace ts {
return instantiateType(baseTrueType, mapper);
}
// Return falseType for a definitely false extends check
if (!typeMaybeAssignableTo(instantiateType(checkType, anyMapper), instantiateType(extendsType, constraintMapper))) {
if (!isTypeAssignableTo(instantiateType(checkType, anyMapper), instantiateType(extendsType, constraintMapper))) {
return instantiateType(baseFalseType, mapper);
}
// Return a deferred type for a check that is neither definitely true nor definitely false
@@ -8704,8 +8700,8 @@ namespace ts {
function getConditionalTypeInstantiation(type: ConditionalType, mapper: TypeMapper): Type {
const target = type.target || type;
const combinedMapper = type.mapper ? combineTypeMappers(type.mapper, mapper) : mapper;
// Check if we have a conditional type of the form T extends U ? X : Y, where T is a type parameter.
// If so, the conditional type is distributive over a union type and when T is instantiated to a union
// Check if we have a conditional type where the check type is a naked type parameter. If so,
// the conditional type is distributive over union types and when T is instantiated to a union
// type A | B, we produce (A extends U ? X : Y) | (B extends U ? X : Y).
const checkType = target.checkType;
if (checkType.flags & TypeFlags.TypeParameter) {