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Add a fastpath for comparing common mapped types like Pick which avoids manufacturing intermediate type identities #39696

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Jul 22, 2020
Merged

Add a fastpath for comparing common mapped types like Pick which avoids manufacturing intermediate type identities #39696

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21 changes: 16 additions & 5 deletions src/compiler/checker.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -16958,12 +16958,23 @@ namespace ts {
if (includeOptional
? !(filteredByApplicability!.flags & TypeFlags.Never)
: isRelatedTo(targetConstraint, sourceKeys)) {
const typeParameter = getTypeParameterFromMappedType(target);
const indexingType = filteredByApplicability ? getIntersectionType([filteredByApplicability, typeParameter]) : typeParameter;
const indexedAccessType = getIndexedAccessType(source, indexingType);
const templateType = getTemplateTypeFromMappedType(target);
if (result = isRelatedTo(indexedAccessType, templateType, reportErrors)) {
return result;
const typeParameter = getTypeParameterFromMappedType(target);

// Fastpath: When the template has the form Obj[P] where P is the mapped type parameter, directly compare `source` with `Obj`
// to avoid creating the (potentially very large) number of new intermediate types made by manufacturing `source[P]`
const nonNullComponent = extractTypesOfKind(templateType, ~TypeFlags.Nullable);
if (nonNullComponent.flags & TypeFlags.IndexedAccess && (nonNullComponent as IndexedAccessType).indexType === typeParameter) {
if (result = isRelatedTo(source, (nonNullComponent as IndexedAccessType).objectType, reportErrors)) {
return result;
}
}
else {
const indexingType = filteredByApplicability ? getIntersectionType([filteredByApplicability, typeParameter]) : typeParameter;
const indexedAccessType = getIndexedAccessType(source, indexingType);
if (result = isRelatedTo(indexedAccessType, templateType, reportErrors)) {
return result;
}
}
}
originalErrorInfo = errorInfo;
Expand Down