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Improve instanceof for structurally identical types

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This commit is contained in:
Anders Hejlsberg 2016-08-08 16:40:59 -07:00
parent 8ea90ab28b
commit 624e52e834
1 changed files with 5 additions and 1 deletions
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6
src/compiler/checker.ts
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@ -8569,8 +8569,12 @@ namespace ts {
}
function getNarrowedType(type: Type, candidate: Type, assumeTrue: boolean) {
// In the false branch we keep types that aren't subtypes of the candidate type and we keep structurally
// identical types except for the current type itself. The latter rule means that given two structurally
// identical classes A and B and a variable x of type A, in the false branch of an 'x instanceof B' type
// guard x keeps type A and is not narrowed to type never.
if (!assumeTrue) {
return filterType(type, t => !isTypeSubtypeOf(t, candidate));
return filterType(type, t => !isTypeSubtypeOf(t, candidate) || isTypeIdenticalTo(t, candidate) && t !== candidate);
}
// If the current type is a union type, remove all constituents that aren't assignable to
// the candidate type. If one or more constituents remain, return a union of those.