Error using non-null-assertion on a method with a `this` parameter (regression)

[whatisaphone]

I hit a regression trying to upgrade to the latest version.

TypeScript Version: Tested with 2.8.1 and 2.9.0-dev.20180330

Search Terms:

• non null assertion
• The 'this' context of type 'void'

Code

interface Foo {
    required: ((this: Foo) => void);
    optional: ((this: Foo) => void) | null;
}

declare const foo: Foo;
foo.required();
foo.optional!();

Expected behavior:

Successful compilation

Actual behavior:

Error on the last line:

The 'this' context of type 'void' is not assignable to method's 'this' of type 'HTMLElement'.

Playground Link:

https://www.typescriptlang.org/play/index.html#src=interface%20Foo%20%7B%0D%0A%20%20%20%20required%3A%20((this%3A%20Foo)%20%3D>%20void)%3B%0D%0A%20%20%20%20optional%3A%20((this%3A%20Foo)%20%3D>%20void)%20%7C%20null%3B%0D%0A%7D%0D%0A%0D%0Adeclare%20const%20foo%3A%20Foo%3B%0D%0Afoo.required()%3B%0D%0Afoo.optional!()%3B

Related Issues:

N/A

[Jessidhia]

By the title I thought it'd be a duplicate of #22934 but doesn't seem (directly) related 🤔

[sandersn]

The check for the callee doesn't skip ! syntax (because it didn't exist at the time), doesn't find that the callee is foo.optional and defaults to void when it can't check it. The fix is to walk down past any !s when looking for the callee.

[sandersn]

Fix is up at #23097