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Tweak union type inference to restore previous behavior

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This commit is contained in:
Anders Hejlsberg
2019-04-05 13:23:54 -07:00
parent 17ceddaa1a
commit f93f8d32c4
1 changed files with 16 additions and 8 deletions
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24
src/compiler/checker.ts
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@@ -14724,17 +14724,25 @@ namespace ts {
inferFromTypes(source, (<ConditionalType>target).falseType);
}
else if (target.flags & TypeFlags.UnionOrIntersection) {
// We infer from types that are not naked type variables first so that inferences we
// make from nested naked type variables and given slightly higher priority by virtue
// of being first in the candidates array.
for (const t of (<UnionOrIntersectionType>target).types) {
const savePriority = priority;
// Inferences directly to naked type variables are given lower priority as they are
// less specific. For example, when inferring from Promise<string> to T | Promise<T>,
// we want to infer string for T, not Promise<string> | string.
if (getInferenceInfoForType(t)) {
priority |= InferencePriority.NakedTypeVariable;
if (!getInferenceInfoForType(t)) {
inferFromTypes(source, t);
}
inferFromTypes(source, t);
priority = savePriority;
}
// Inferences directly to naked type variables are given lower priority as they are
// less specific. For example, when inferring from Promise<string> to T | Promise<T>,
// we want to infer string for T, not Promise<string> | string.
const savePriority = priority;
priority |= InferencePriority.NakedTypeVariable;
for (const t of (<UnionOrIntersectionType>target).types) {
if (getInferenceInfoForType(t)) {
inferFromTypes(source, t);
}
}
priority = savePriority;
}
else if (source.flags & TypeFlags.Union) {
// Source is a union or intersection type, infer from each constituent type