Weak type errors on signature-only types

[sandersn]

Now source types that only have a call signature (like functions) or construct signature will get a weak type error too. This is really good for catching uncalled functions:

functionTakingWeakType(returnWeakType);
// OOPS. Forgot to call `returnWeakType()`. That's an error!

Fixes #17656

Previously I didn't think enough about these types and assumed that we couldn't catch errors like this without breaking something else. Nothing breaks in our test suite, though, so I'm going to run it on DefinitelyTyped and see what breaks.

[sandersn]

Definitely Typed didn't turn up any new breaks. That's good, although a little surprising that it didn't find any mistakes in the tests anywhere.

[weswigham]

Nice addition, but could we also add an extra bit to the error where we check if the function or ctor return type is compatible with the weak type, and if so, then suggest calling/constructing it? (And maybe give it a quick fix if it takes no arguments?)

[sandersn]

"Did you forget to call this?"

Sure, I can do that.

[sandersn]

I think I'll merge without a quick fix. I don't think it's high-value when it only applies to nullary functions, and quick fixes are quite finicky to get right. I can do it after the 2.5 release if we decide it's worth it.

[weswigham]

Alright, sounds fine.

[weswigham]

Hmmm... Its a little odd to reuse call for the constructor version. Maybe a version that says "Did you forget to construct this with new?". I'm not too hung up on it, its just that the fix implied by the suggestion would lead to another error.

[sandersn]

I thought about that and decided I'd rather make the implementation a little simpler and the error slightly less accurate.