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Merged
ahejlsberg merged 2 commits into from
Jan 6, 2023
Merged

Improve logic that chooses co- vs. contra-variant inferences #52123

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7ae62dc
Improve logic that chooses co- vs. contra-variant inferences
ahejlsberg Jan 6, 2023
26902df
Add tests
ahejlsberg Jan 6, 2023
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Improve logic that chooses co- vs. contra-variant inferences
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@ahejlsberg
ahejlsberg committed Jan 6, 2023
commit 7ae62dc1e7a68cc2bf8d01b9a8b07269b8106606
14 changes: 9 additions & 5 deletions src/compiler/checker.ts
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Expand Up @@ -24504,11 +24504,15 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
if (signature) {
const inferredCovariantType = inference.candidates ? getCovariantInference(inference, signature) : undefined;
if (inference.contraCandidates) {
// If we have both co- and contra-variant inferences, we prefer the contra-variant inference
// unless the co-variant inference is a subtype of some contra-variant inference and not 'never'.
inferredType = inferredCovariantType && !(inferredCovariantType.flags & TypeFlags.Never) &&
some(inference.contraCandidates, t => isTypeSubtypeOf(inferredCovariantType, t)) ?
inferredCovariantType : getContravariantInference(inference);
// If we have both co- and contra-variant inferences, we use the co-variant inference if it is not 'never',
// it is a subtype of some contra-variant inference, and no other type parameter is constrained to this type
// parameter and has inferences that would conflict. Otherwise, we use the contra-variant inference.
const useCovariantType = inferredCovariantType && !(inferredCovariantType.flags & TypeFlags.Never) &&
some(inference.contraCandidates, t => isTypeSubtypeOf(inferredCovariantType, t)) &&
every(context.inferences, other => other === inference ||
getConstraintOfTypeParameter(other.typeParameter) !== inference.typeParameter ||
every(other.candidates, t => isTypeSubtypeOf(t, inferredCovariantType)));
inferredType = useCovariantType ? inferredCovariantType : getContravariantInference(inference);
}
else if (inferredCovariantType) {
inferredType = inferredCovariantType;
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