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Make never inferences with template literal types only in special cases (#46075)

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* Make 'never' inferences with template literal types only in special cases

* Accept new baselines

* Add regression test

* Fix comment
This commit is contained in:
Anders Hejlsberg
2021-09-27 06:47:47 -07:00
committed by GitHub
parent 4ce902c5fa
commit 26aef89a72
7 changed files with 85 additions and 7 deletions
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12
src/compiler/checker.ts
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@@ -21939,8 +21939,16 @@ namespace ts {
function inferToTemplateLiteralType(source: Type, target: TemplateLiteralType) {
const matches = inferTypesFromTemplateLiteralType(source, target);
const types = target.types;
for (let i = 0; i < types.length; i++) {
inferFromTypes(matches ? matches[i] : neverType, types[i]);
// When the target template literal contains only placeholders (meaning that inference is intended to extract
// single characters and remainder strings) and inference fails to produce matches, we want to infer 'never' for
// each placeholder such that instantiation with the inferred value(s) produces 'never', a type for which an
// assignment check will fail. If we make no inferences, we'll likely end up with the constraint 'string' which,
// upon instantiation, would collapse all the placeholders to just 'string', and an assignment check might
// succeed. That would be a pointless and confusing outcome.
if (matches || every(target.texts, s => s.length === 0)) {
for (let i = 0; i < types.length; i++) {
inferFromTypes(matches ? matches[i] : neverType, types[i]);
}
}
}