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Improve logic that chooses co- vs. contra-variant inferences (#57909)

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Mateusz Burzyński
2024-06-18 00:25:06 +02:00
committed by GitHub
parent ed81ca6c0a
commit a71841c77b
7 changed files with 305 additions and 5 deletions
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12
src/compiler/checker.ts
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@@ -26811,15 +26811,17 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
const inferredContravariantType = inference.contraCandidates ? getContravariantInference(inference) : undefined;
if (inferredCovariantType || inferredContravariantType) {
// If we have both co- and contra-variant inferences, we prefer the co-variant inference if it is not 'never',
// all co-variant inferences are subtypes of it (i.e. it isn't one of a conflicting set of candidates), it is
// a subtype of some contra-variant inference, and no other type parameter is constrained to this type parameter
// all co-variant inferences are assignable to it (i.e. it isn't one of a conflicting set of candidates), it is
// assignable to some contra-variant inference, and no other type parameter is constrained to this type parameter
// and has inferences that would conflict. Otherwise, we prefer the contra-variant inference.
// Similarly ignore co-variant `any` inference when both are available as almost everything is assignable to it
// and it would spoil the overall inference.
const preferCovariantType = inferredCovariantType && (!inferredContravariantType ||
!(inferredCovariantType.flags & TypeFlags.Never) &&
some(inference.contraCandidates, t => isTypeSubtypeOf(inferredCovariantType, t)) &&
!(inferredCovariantType.flags & (TypeFlags.Never | TypeFlags.Any)) &&
some(inference.contraCandidates, t => isTypeAssignableTo(inferredCovariantType, t)) &&
every(context.inferences, other =>
other !== inference && getConstraintOfTypeParameter(other.typeParameter) !== inference.typeParameter ||
every(other.candidates, t => isTypeSubtypeOf(t, inferredCovariantType))));
every(other.candidates, t => isTypeAssignableTo(t, inferredCovariantType))));
inferredType = preferCovariantType ? inferredCovariantType : inferredContravariantType;
fallbackType = preferCovariantType ? inferredContravariantType : inferredCovariantType;
}