Generic constrained to function: parameter is incorrectly inferred as any

[OliverJAsh]

TypeScript Version: 4.0.2

Search Terms:

Code

declare function useCallback<T extends (...args: any[]) => any>(
    callback: T,
): T;
const test: ((x: string) => void) | undefined = useCallback((x) => {
    x; // ❌ type is inferred as `any`, expected `string`
});

For context, this typing of useCallback is extracted from the React types.

Expected behavior: see above

Actual behavior: see above

Playground Link: https://www.typescriptlang.org/play?#code/CYUwxgNghgTiAEAzArgOzAFwJYHtX2QGcQBhKCCAIyjAGsAeAFXhAA8MRVhD4AKAOkGwA5oQBc8KKgCeAbQC6ASngBeAHySZa3gCh4++GHJUatCYwA0OxeYDcOsHkIZ4HZxN69WE5zCyphZXV4ADccLGBlAB8CLhBEfxBgVQJiMgpqOk9WII0Abz0DVlt4AHpS+EAZcldpAAcELB5-RBAYOGSoHgADKWkuixZWeswk+C7ff2EunQBfRVsgA

Related Issues:

• Cannot infer generic argument type from passed callback #31146
• Add type definitions for the proposed React Hooks API DefinitelyTyped/DefinitelyTyped#30057 (comment)

[RyanCavanaugh]

The problem here is the contextual type being undefined | (x: string) => void instead of just (x: string) => void - that defeats our matching logic for using the return type as an inference candidate for T

[Andarist]

It's likely related to this: #50719 (comment)