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Only infer to single naked type parameters in intersections

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This commit is contained in:
Anders Hejlsberg
2019-04-10 17:41:51 -10:00
parent ff959096df
commit ca47ddb583
1 changed files with 15 additions and 8 deletions
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23
src/compiler/checker.ts
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@@ -14773,22 +14773,29 @@ namespace ts {
// 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.
let typeVariableCount = 0;
for (const t of (<UnionOrIntersectionType>target).types) {
if (!getInferenceInfoForType(t)) {
if (getInferenceInfoForType(t)) {
typeVariableCount++;
}
else {
inferFromTypes(source, t);
}
}
// 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);
// we want to infer string for T, not Promise<string> | string. For intersection types
// we only infer to single naked type variables.
if (target.flags & TypeFlags.Union ? typeVariableCount !== 0 : typeVariableCount === 1) {
const savePriority = priority;
priority |= InferencePriority.NakedTypeVariable;
for (const t of (<UnionOrIntersectionType>target).types) {
if (getInferenceInfoForType(t)) {
inferFromTypes(source, t);
}
}
priority = savePriority;
}
priority = savePriority;
}
else if (source.flags & TypeFlags.Union) {
// Source is a union or intersection type, infer from each constituent type