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Optimize T[K] where T has string index signature and no other members

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This commit is contained in:
Anders Hejlsberg 2019-04-08 09:36:34 -10:00
parent 71f3e1fdde
commit eb1e7ec6db
1 changed files with 11 additions and 0 deletions
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11
src/compiler/checker.ts
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@ -10145,6 +10145,11 @@ namespace ts {
if (objectType === wildcardType || indexType === wildcardType) {
return wildcardType;
}
// If the object type has a string index signature and no other members we know that the result will
// always be the type of that index signature and we can simplify accordingly.
if (isStringIndexSignatureOnlyType(objectType) && !(indexType.flags & TypeFlags.Nullable) && isTypeAssignableToKind(indexType, TypeFlags.String | TypeFlags.Number)) {
indexType = stringType;
}
// If the index type is generic, or if the object type is generic and doesn't originate in an expression,
// we are performing a higher-order index access where we cannot meaningfully access the properties of the
// object type. Note that for a generic T and a non-generic K, we eagerly resolve T[K] if it originates in
@ -12022,6 +12027,12 @@ namespace ts {
return !!(getObjectFlags(type) & ObjectFlags.Anonymous) && isEmptyObjectType(type);
}
function isStringIndexSignatureOnlyType(type: Type): boolean {
return type.flags & TypeFlags.Object && getPropertiesOfType(type).length === 0 && getIndexInfoOfType(type, IndexKind.String) && !getIndexInfoOfType(type, IndexKind.Number) ||
type.flags & TypeFlags.UnionOrIntersection && every((<UnionOrIntersectionType>type).types, isStringIndexSignatureOnlyType) ||
false;
}
function isEnumTypeRelatedTo(sourceSymbol: Symbol, targetSymbol: Symbol, errorReporter?: ErrorReporter) {
if (sourceSymbol === targetSymbol) {
return true;