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Infer to partially homomorphic mapped types (such as Pick<T, K>)

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This commit is contained in:
Anders Hejlsberg 2019-02-06 14:23:49 -08:00
parent 2f9218f346
commit 52a9cfb0a9
1 changed files with 29 additions and 16 deletions
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45
src/compiler/checker.ts
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@ -14570,11 +14570,24 @@ namespace ts {
return undefined;
}
function inferFromMappedTypeConstraint(source: Type, target: Type, constraintType: Type): boolean {
function inferToHomomorphicMappedType(source: Type, target: MappedType, constraintType: IndexType) {
const inference = getInferenceInfoForType(constraintType.type);
if (inference && !inference.isFixed) {
const inferredType = inferTypeForHomomorphicMappedType(source, target, constraintType);
if (inferredType) {
const savePriority = priority;
priority |= InferencePriority.HomomorphicMappedType;
inferFromTypes(inferredType, inference.typeParameter);
priority = savePriority;
}
}
}
function inferToMappedType(source: Type, target: MappedType, constraintType: Type): boolean {
if (constraintType.flags & TypeFlags.Union) {
let result = false;
for (const type of (constraintType as UnionType).types) {
result = inferFromMappedTypeConstraint(source, target, type) || result;
result = inferToMappedType(source, target, type) || result;
}
return result;
}
@ -14583,31 +14596,31 @@ namespace ts {
// where T is a type variable. Use inferTypeForHomomorphicMappedType to infer a suitable source
// type and then make a secondary inference from that type to T. We make a secondary inference
// such that direct inferences to T get priority over inferences to Partial<T>, for example.
const inference = getInferenceInfoForType((<IndexType>constraintType).type);
if (inference && !inference.isFixed) {
const inferredType = inferTypeForHomomorphicMappedType(source, <MappedType>target, constraintType as IndexType);
if (inferredType) {
const savePriority = priority;
priority |= InferencePriority.HomomorphicMappedType;
inferFromTypes(inferredType, inference.typeParameter);
priority = savePriority;
}
}
inferToHomomorphicMappedType(source, target, <IndexType>constraintType);
return true;
}
if (constraintType.flags & TypeFlags.TypeParameter) {
// We're inferring from some source type S to a mapped type { [P in T]: X }, where T is a type
// parameter. Infer from 'keyof S' to T and infer from a union of each property type in S to X.
// We're inferring from some source type S to a mapped type { [P in K]: X }, where K is a type
// parameter. First infer from 'keyof S' to K.
const savePriority = priority;
priority |= InferencePriority.MappedTypeConstraint;
inferFromTypes(getIndexType(source), constraintType);
priority = savePriority;
// If K is constrained to an index type keyof T, where T is a type parameter, proceed to make
// the same inferences as we would for a homomorphic mapped type { [P in keyof T]: X } (this
// enables us to make meaningful inferences when the target is a Pick<T, K>). Otherwise, infer
// from a union of the property types in the source to the template type X.
const extendedConstraint = getConstraintOfType(constraintType);
if (extendedConstraint && extendedConstraint.flags & TypeFlags.Index) {
inferToHomomorphicMappedType(source, target, <IndexType>extendedConstraint);
return true;
}
const valueTypes = compact([
getIndexTypeOfType(source, IndexKind.String),
getIndexTypeOfType(source, IndexKind.Number),
...map(getPropertiesOfType(source), getTypeOfSymbol)
]);
inferFromTypes(getUnionType(valueTypes), getTemplateTypeFromMappedType(<MappedType>target));
inferFromTypes(getUnionType(valueTypes), getTemplateTypeFromMappedType(target));
return true;
}
return false;
@ -14622,7 +14635,7 @@ namespace ts {
}
if (getObjectFlags(target) & ObjectFlags.Mapped) {
const constraintType = getConstraintTypeFromMappedType(<MappedType>target);
if (inferFromMappedTypeConstraint(source, target, constraintType)) {
if (inferToMappedType(source, <MappedType>target, constraintType)) {
return;
}
}