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Experiment short-circuiting of union distribution #50640

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Experiment short-circuiting of union distribution #50640

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Experiment short-circuiting of union distribution
Zzzen Sep 5, 2022
8f978fd
Merge branch 'main' into short-circuiting-of-union-type
jakebailey Jan 27, 2023
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36 changes: 33 additions & 3 deletions src/compiler/checker.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -541,6 +541,7 @@ import {
isImportTypeNode,
isIndexedAccessTypeNode,
isInExpressionContext,
isInferTypeNode,
isInfinityOrNaNString,
isInitializedProperty,
isInJSDoc,
Expand Down Expand Up @@ -18719,16 +18720,45 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
// Distributive conditional types are distributed over union types. For example, when the
// distributive conditional type T extends U ? X : Y is instantiated with A | B for T, the
// result is (A extends U ? X : Y) | (B extends U ? X : Y).
result = distributionType && checkType !== distributionType && distributionType.flags & (TypeFlags.Union | TypeFlags.Never) ?
mapTypeWithAlias(getReducedType(distributionType), t => getConditionalType(root, prependTypeMapping(checkType, t, newMapper)), aliasSymbol, aliasTypeArguments) :
getConditionalType(root, newMapper, aliasSymbol, aliasTypeArguments);
result = distributionType && checkType !== distributionType && distributionType.flags & (TypeFlags.Union | TypeFlags.Never)
? root.isDistributive && !isDistributionDependent(root) && !containsInferTypeNode(root.node)
? getConditionalTypeForSimpleDistributionType(getReducedType(distributionType), (t) => getConditionalType(root, prependTypeMapping(checkType, t, newMapper)), aliasSymbol, aliasTypeArguments)
: mapTypeWithAlias(getReducedType(distributionType), (t) => getConditionalType(root, prependTypeMapping(checkType, t, newMapper)), aliasSymbol, aliasTypeArguments)
: getConditionalType(root, newMapper, aliasSymbol, aliasTypeArguments);
root.instantiations!.set(id, result);
}
return result;
}
return type;
}

// For types like A extends B ? C : D, if the check-type (A) appears in neither branch and the extends type (B) has no infer positions,
// the output type will be combinations of C and D.
function getConditionalTypeForSimpleDistributionType(type: Type, mapper: (t: Type) => Type, aliasSymbol: Symbol | undefined, aliasTypeArguments: readonly Type[] | undefined): Type {
if (!(type.flags & TypeFlags.Union && aliasSymbol)) {
return mapType(type, mapper);
}
let firstMappedType: Type | undefined;
for(const t of (type as UnionType).types) {
const mapped = mapper(t);
if (mapped.flags & TypeFlags.AnyOrUnknown) {
return mapped;
}
else if (!firstMappedType) {
firstMappedType = mapped;
}
else if (firstMappedType !== mapped) {
return getUnionType([firstMappedType, mapped], UnionReduction.Literal, aliasSymbol, aliasTypeArguments);
}
}
Debug.assertIsDefined(firstMappedType);
return firstMappedType;
}

function containsInferTypeNode(node: Node) {
return isInferTypeNode(node) || forEachChildRecursively(node, isInferTypeNode);
}

function instantiateType(type: Type, mapper: TypeMapper | undefined): Type;
function instantiateType(type: Type | undefined, mapper: TypeMapper | undefined): Type | undefined;
function instantiateType(type: Type | undefined, mapper: TypeMapper | undefined): Type | undefined {
Expand Down