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Use strict subtype relation in getCommonSupertype #61903

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Merged
ahejlsberg merged 4 commits into from
Jun 24, 2025
Merged

Use strict subtype relation in getCommonSupertype #61903

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b7439c1
Use strict subtype relation in getCommonSupertype
ahejlsberg Jun 19, 2025
a6d3453
Find single strict supertype, otherwise use assignability relation
ahejlsberg Jun 20, 2025
544f4d7
Try to find single strict supertype, otherwise do previous behavior
ahejlsberg Jun 20, 2025
cb554a8
Fix lint issue and add comment
ahejlsberg Jun 20, 2025
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12 changes: 11 additions & 1 deletion src/compiler/checker.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -25340,11 +25340,21 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
// right is a supertype.
const superTypeOrUnion = literalTypesWithSameBaseType(primaryTypes) ?
getUnionType(primaryTypes) :
reduceLeft(primaryTypes, (s, t) => isTypeSubtypeOf(s, t) ? t : s)!;
getSingleCommonSupertype(primaryTypes);
// Add any nullable types that occurred in the candidates back to the result.
return primaryTypes === types ? superTypeOrUnion : getNullableType(superTypeOrUnion, getCombinedTypeFlags(types) & TypeFlags.Nullable);
}

function getSingleCommonSupertype(types: Type[]) {
// First, find the leftmost type for which no type to the right is a strict supertype, and if that
// type is a strict supertype of all other candidates, return it. Otherwise, return the leftmost type
// for which no type to the right is a (regular) supertype.
const candidate = reduceLeft(types, (s, t) => isTypeStrictSubtypeOf(s, t) ? t : s)!;
return every(types, t => t === candidate || isTypeStrictSubtypeOf(t, candidate)) ?
candidate :
reduceLeft(types, (s, t) => isTypeSubtypeOf(s, t) ? t : s)!;
}

// Return the leftmost type for which no type to the right is a subtype.
function getCommonSubtype(types: Type[]) {
return reduceLeft(types, (s, t) => isTypeSubtypeOf(t, s) ? t : s)!;
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