Generic argument inference no longer working for specific case

TypeScript Version: >3.9.2

Search Terms: generic argument inference TS2345

Code

declare namespace React {
    type WeakValidationMap<T> = {
        [K in keyof T]?: null extends T[K] ? string : string
    };

    interface FunctionComponent<P = {}> {
        propTypes?: WeakValidationMap<P>;
    }
}

type A<T1> = <T2>() => React.FunctionComponent<T1 & T2>;

function B<T>(_: A<T>) {}

interface C {
    r: String;
}

function myFunction<T2>(): React.FunctionComponent<C & T2> {
    return {};
}

B(myFunction) // Error
B<C>(myFunction) // No error

Expected behavior: In v3.8.3, the code above did not have any compilation error.

Actual behavior: Compilation error TS2345 when I do not explicitly pass the generic type.

Playground Link: Link

Note: I use React, here I only included the minimal React interfaces to reproduce the issue. The issue is also reproducible with React code directly here

Looks to be caused by #37261 - the erased signature seems ok, in theory, but because it erases to any, any & T1 becomes just any, and we lose the ability to infer to T1. If we erase to unknown instead, this should work - unknown will still behave like any in unions still, though (and prevent inference to other union members), which makes me wonder if erasing to a unique anti-any-type-thing might be better - something which evaporates in both unions and intersections, but otherwise behaves anyish.

I'm wondering if this is the same issue (also works in 3.8 and fails in 3.9):

interface Input<V> { v: V }
const inner = <V>(fn: <T extends Input<V>>(t: T) => null) => null;
export default () => inner(<T extends Input<string>>(t: T) => null); // <-- error here

Calling inner<string>(...) explicitly works.

This shorter example has the same general structure (an argument which is a function with a complicated parameter), but there's too much going on for me to be sure it's the same bug.

This behaves as expected now

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Generic argument inference no longer working for specific case #39080

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Generic argument inference no longer working for specific case #39080

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weswigham commented on Jun 23, 2020

davidje13 commented on Jul 15, 2020

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RyanCavanaugh commented on Feb 15, 2023

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