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Preserve type parameters in generic contextual pure function types

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Anders Hejlsberg
2017-06-03 16:52:38 -07:00
parent dbf036298d
commit 9e613b9fd5
1 changed files with 15 additions and 5 deletions
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20
src/compiler/checker.ts
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@@ -14959,11 +14959,21 @@ namespace ts {
// We clone the contextual mapper to avoid disturbing a resolution in progress for an
// outer call expression. Effectively we just want a snapshot of whatever has been
// inferred for any outer call expression so far.
const mapper = cloneTypeMapper(getContextualMapper(node));
const instantiatedType = instantiateType(contextualType, mapper);
const returnType = getReturnTypeOfSignature(signature);
// Inferences made from return types have lower priority than all other inferences.
inferTypes(context.inferences, instantiatedType, returnType, InferencePriority.ReturnType);
const instantiatedType = instantiateType(contextualType, cloneTypeMapper(getContextualMapper(node)));
// If the contextual type is a generic pure function type, we instantiate the type with
// its own type parameters and type arguments. This ensures that the type parameters are
// not erased to type any during type inference such that they can be inferred as actual
// types from the contextual type. For example:
// declare function arrayMap<T, U>(f: (x: T) => U): (a: T[]) => U[];
// const boxElements: <A>(a: A[]) => { value: A }[] = arrayMap(value => ({ value }));
// Above, the type of the 'value' parameter is inferred to be 'A'.
const contextualSignature = getSingleCallSignature(instantiatedType);
const inferenceSourceType = contextualSignature && contextualSignature.typeParameters ?
getOrCreateTypeFromSignature(getSignatureInstantiation(contextualSignature, contextualSignature.typeParameters)) :
instantiatedType;
const inferenceTargetType = getReturnTypeOfSignature(signature);
// Inferences made from return types have lower priority than all other inferences.
inferTypes(context.inferences, inferenceSourceType, inferenceTargetType, InferencePriority.ReturnType);
}
}