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Better scheme for choosing between co- and contra-variant inferences

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This commit is contained in:
Anders Hejlsberg
2018-09-10 11:25:30 -07:00
parent cf2f339a5c
commit 46bd405e59
1 changed files with 10 additions and 8 deletions
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18
src/compiler/checker.ts
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@@ -13752,15 +13752,17 @@ namespace ts {
if (!inferredType) {
const signature = context.signature;
if (signature) {
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;
const inferredCovariantType = inference.candidates ? getCovariantInference(inference, signature) : undefined;
if (inference.contraCandidates) {
const inferredContravariantType = getContravariantInference(inference);
// If we have both co- and contra-variant inferences, we prefer the contra-variant inference
// unless the co-variant inference is a subtype and not 'never'.
inferredType = inferredCovariantType && !(inferredCovariantType.flags & TypeFlags.Never) &&
isTypeSubtypeOf(inferredCovariantType, inferredContravariantType) ?
inferredCovariantType : inferredContravariantType;
}
if (inference.candidates) {
inferredType = getCovariantInference(inference, signature);
else if (inferredCovariantType) {
inferredType = inferredCovariantType;
}
else if (context.flags & InferenceFlags.NoDefault) {
// We use silentNeverType as the wildcard that signals no inferences.