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Covariant inferences (other than never) preferred over contravariant

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This commit is contained in:
Anders Hejlsberg
2018-08-20 16:04:11 -07:00
parent 435a12e476
commit 15eaba254b
1 changed files with 5 additions and 4 deletions
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9
src/compiler/checker.ts
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@@ -13638,10 +13638,11 @@ namespace ts {
if (!inferredType) {
const signature = context.signature;
if (signature) {
if (inference.contraCandidates) {
// If we have contravariant inferences we find the best common subtype and treat
// that as a single covariant candidate.
inference.candidates = append(inference.candidates, getContravariantInference(inference));
if (inference.contraCandidates && (!inference.candidates || inference.candidates.length === 1 && inference.candidates[0].flags & TypeFlags.Never)) {
// If we have contravariant inferences, but no covariant inferences or a single
// covariant inference of 'never', we find the best common subtype and treat that
// as a single covariant candidate.
inference.candidates = [getContravariantInference(inference)];
inference.contraCandidates = undefined;
}
if (inference.candidates) {