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Merge pull request #26425 from mattmccutchen/issue-26405

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Go back to old narrowing algorithm but don't narrow in cases like #26130
This commit is contained in:
Anders Hejlsberg
2018-08-20 15:18:29 -07:00
committed by GitHub
parent 435a12e476 cc1c2ab6b2
commit 103f894d38
7 changed files with 78 additions and 6 deletions
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20
src/compiler/checker.ts
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@@ -13900,6 +13900,18 @@ namespace ts {
return flow.id;
}
function typeMaybeAssignableTo(source: Type, target: Type) {
if (!(source.flags & TypeFlags.Union)) {
return isTypeAssignableTo(source, target);
}
for (const t of (<UnionType>source).types) {
if (isTypeAssignableTo(t, target)) {
return true;
}
}
return false;
}
// Remove those constituent types of declaredType to which no constituent type of assignedType is assignable.
// For example, when a variable of type number | string | boolean is assigned a value of type number | boolean,
// we remove type string.
@@ -13908,8 +13920,12 @@ namespace ts {
if (assignedType.flags & TypeFlags.Never) {
return assignedType;
}
const reducedType = filterType(declaredType, t => isTypeComparableTo(assignedType, t));
if (!(reducedType.flags & TypeFlags.Never)) {
const reducedType = filterType(declaredType, t => typeMaybeAssignableTo(assignedType, t));
// Our crude heuristic produces an invalid result in some cases: see GH#26130.
// For now, when that happens, we give up and don't narrow at all. (This also
// means we'll never narrow for erroneous assignments where the assigned type
// is not assignable to the declared type.)
if (isTypeAssignableTo(assignedType, reducedType)) {
return reducedType;
}
}