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When instantiating a mapped type, clone the type parameter. (#27597)

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This gives the type parameter returned by getTypeParameterFromMappedType
an accurate constraint.

Fixes #27596.
This commit is contained in:
Matt McCutchen
2018-10-11 18:45:51 -04:00
committed by Wesley Wigham
parent 4d504f9b30
commit d19fb98ec6
5 changed files with 64 additions and 1 deletions
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8
src/compiler/checker.ts
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@@ -6850,7 +6850,7 @@ namespace ts {
function getConstraintTypeFromMappedType(type: MappedType) {
return type.constraintType ||
(type.constraintType = instantiateType(getConstraintOfTypeParameter(getTypeParameterFromMappedType(type)), type.mapper || identityMapper) || errorType);
(type.constraintType = getConstraintOfTypeParameter(getTypeParameterFromMappedType(type)) || errorType);
}
function getTemplateTypeFromMappedType(type: MappedType) {
@@ -10413,6 +10413,12 @@ namespace ts {
const result = <AnonymousType>createObjectType(type.objectFlags | ObjectFlags.Instantiated, type.symbol);
if (type.objectFlags & ObjectFlags.Mapped) {
(<MappedType>result).declaration = (<MappedType>type).declaration;
// C.f. instantiateSignature
const origTypeParameter = getTypeParameterFromMappedType(<MappedType>type);
const freshTypeParameter = cloneTypeParameter(origTypeParameter);
(<MappedType>result).typeParameter = freshTypeParameter;
mapper = combineTypeMappers(makeUnaryTypeMapper(origTypeParameter, freshTypeParameter), mapper);
freshTypeParameter.mapper = mapper;
}
result.target = type;
result.mapper = mapper;