Less aggressive contextual signature instantiation #33228
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weswigham
reviewed
Sep 9, 2019
| @@ -19193,7 +19194,7 @@ namespace ts { | |||
| getContextualTypeForObjectLiteralMethod(node, contextFlags) : | |||
| getContextualType(node, contextFlags); | |||
| const instantiatedType = instantiateContextualType(contextualType, node, contextFlags); | |||
| if (instantiatedType) { | |||
| if (instantiatedType && !(contextFlags && contextFlags & ContextFlags.NoConstraints && instantiatedType.flags & TypeFlags.TypeVariable)) { | |||
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In addition to TypeVariable, should this also be looking for Substitution (since that wraps a type variable pretty directly)?
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Can you write an example that would depend on this?
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declare function foo<T extends (x: string) => string>(f: T): T;
type SubstituteOf<T> = T extends {} ? T : never;
const f = foo(<U>(x: SubstituteOf<U>) => x); // <U>(x: U) => Ustill instantiates as string on this PR.
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No, for your example you get what you'd expect with the PR:
const f = foo(<U>(x: SubstituteOf<U>) => x); // <U>(x: SubstituteOf<U>) => SubstituteOf<U>The substitution type would need to be in the contextual type (i.e. the parameter's type would need to be a substitution type) for the original question you posed to make sense. I don't see how that could happen which is why I asked for an example.
RyanCavanaugh
approved these changes
Sep 16, 2019
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With this PR we perform less aggressive contextual signature instantiation:
Previously, we'd infer type
(x: string) => stringforfbecause we'd obtain the apparent type ofTand then instantiate the argument type<U>(x: U) => Uin the context of that type. While not incorrect, a better course of action is to leave the argument type alone and just infer it as-is when inferring directly to a type variable. That's the change in this PR.This change also fixes #32976 because we no longer need to check for mixin constructor types (which wasn't really a correct check to begin with).
Fixes #32976.