[Typechecker] Warn when casting a function type to an existential or archetype type

[theblixguy]

This PR extends the unrelated downcast warning to cast from a function type to an existential type or a archetype type. If the referenced function has a non-void return type, then a fix-it is offered to call the function instead.

Resolves SR-6022.
Resolves rdar://problem/25723751

[theblixguy]

cc @jrose-apple

[slavapestov]

This is not true. A function type can be unconditionally cast to an 'Any'.

It can also be conditionally cast to a generic parameter with no constraints; the cast will succeed if that generic parameter happens to be bound to an identical function type at runtime.

[theblixguy]

@slavapestov Oh right, I should update my comment. If you cast it to Any, then you won't hit this check, because that would've already been handled before this code is called, but I think I should still add a check for !toType->isAny() anyway just to be safe!

This is only for a force-cast and not an conditional cast, but I can extend it to cover the latter scenario.

[slavapestov]

Can you add an assert that you don't get an 'Any' here, and make sure the tests cover the correct behavior?

Conditional-cast and force-cast have the same semantics regarding what casts can ever succeed or not.

So a conditional-cast of a function type to a non-Any existential should also warn that it will always fail.

Similarly, both a force-cast and a conditional-cast of a function type to an unconstrained archetype is OK.

[jrose-apple]

AnyObject should also behave like Any, since it's basically "Any but box it up if you have to".

[theblixguy]

@slavapestov Done, does it look good to you?

[theblixguy]

@jrose-apple That's already handled, so you will get the forced cast from '() -> Any' to 'AnyObject' always succeeds; did you mean to use 'as'? diagnostic.

[jrose-apple]

Running tests for you, still waiting on Slava for final OK.

@swift-ci Please test

[theblixguy]

Thanks Jordan, all tests have passed 🎉

@slavapestov @xedin does this look okay to you?

[theblixguy]

Bump. Is this good to merge now?

[theblixguy]

@slavapestov Can you invoke the CI again?

[xedin]

@swift-ci please test

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 77300c3

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 77300c3

[theblixguy]

OSX build failed with <unknown>:0: error: no group info found for file: 'main.swift' 🤔 Weird. Could someone re-run the OSX test?

[slavapestov]

@swift-ci Please smoke test macOS

[theblixguy]

The smoke test passed! Do we need to run a full OSX test as well or is this good to merge without it?