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Type parameter inference regression in 3.4 #30442
Description
TypeScript Version: 3.4.0-rc
Search Terms: Type parameters, generics, inference, regression
Code
const foo = <T>() => {};
const assign = <T, U>(a: T, b: U) => Object.assign(a, b);
const result: (() => void) & { foo: any } = assign(() => {}, { foo });Expected behavior:
No error
Actual behavior:
Argument of type '{ foo: <T>() => void; }' is not assignable to parameter of type '(() => void) & { foo: any; }'.
Type '{ foo: <T>() => void; }' is not assignable to type '() => void'.
Type '{ foo: <T>() => void; }' provides no match for the signature '(): void'.
Related Issues:
Removing <T> from foo makes this code valid.
Activity
ark120202 commentedon Apr 2, 2019
Reproducible in 3.4.1
RyanCavanaugh commentedon Apr 9, 2019
Anders Hejlsberg (@ahejlsberg) contextual typing strikes again? Moving the initializer to its own expression resolves its problem
ahejlsberg commentedon Apr 10, 2019
This is an effect of #29847. Previously we'd only make inferences to single naked type parameters in unions and intersections. We now infer to each naked type parameter. That works well for union types, but not for intersection types as this example illustrates. I think we should revert to the previous behavior for intersection types and only infer to multiple naked type parameters in union types.