[CSDiagnostics] Diagnose invalid partial application

[xedin]

Detect and diagnose invalid partial application of 'mutating' methods and constructor delegation.

This used to be diagnosed by MiscDiagnostics when invalid solution is already applied to AST,
moving these diagnostics to constraint system allows to detect failures like that early and makes
other diagnostics easier e.g. missing or extraneous metatype base in member reference.

[xedin]

/cc @theblixguy

[xedin]

@jckarter Since you have worked on this before could you take a look? Any other examples I should add?

[xedin]

@swift-ci please test

[jckarter]

Hopefully our existing test cases cover all the edge cases. One of the tricky ones is partially applying an instance method as a static member:

struct X {
  mutating func foo() {}
}

var x = X()
_ = X.foo // an illegal partial application of type (inout X) -> () -> ()
_ = X.foo(&x) // an illegal partial application of type () -> ()
_ = X.foo(&x)() // legal, i guess

I think our existing test cases cover this, but it'd be good to confirm.

[xedin]

@jckarter Yes, existing tests in test/Constraints/mutating_memeber_{compat}.swift check exactly that, I have also added a couple new ones where base is overloaded

[xedin]

@swift-ci please test source compatibility

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 65d18afbe721321033c33339f4b4c40963f7cc89

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 65d18afbe721321033c33339f4b4c40963f7cc89

[slavapestov]

@xedin Also it would be cool to diagnose construction of non-constant metatype values early. Eg if I write

let foo = Int.self
foo(0)

Right now we solve the solution and then diagnose it in MiscDiagnostics.

Same with missing .self:

foo(_: Any.Type) {}
foo(Int)

[xedin]

@slavapestov I'm preparing the PR for that and another partial application problems like:

class A {}

enum B {
  func foo() {
    bar(A()) // should diagnose as `'A' cannot be passed as 'self' parameter to instance method 'bar' of enum 'B'`
  }

  func bar(_: A) {}
}

I'm just trying to figure out the best way to attach fixes to overload choices.

[slavapestov]

Awesome!