[ConstraintSystem] Accept trailing closure if multiple defaulted parameters after last function parameter

[omochi]

This patch improve trailing closure matching with parameters.

Currently, trailing closure can match with function parameter followed by one defaulted parameter.
But it can not do if:

• function type from generic parameter.
• More than one defaulted parameters exist.

For example with below,

  func foo<T>(fn: () -> T, value: Int = 0) {}
  foo { 42 }

  func bar<T>(type: T.Type, fn: T, a: Int = 0) {}
  bar(type: (() -> Int).self) { 42 }

  func baz(fn: () -> Int, a: Int = 0, b: Int = 0, c: Int = 0) {}
  baz { 42 }

Only foo is ok but bar and baz are rejected.

This patch supports all of them.

@xedin Please review it.

[xedin]

Thank you for the contribution, @omochi! Changes make sense to me, I think we just need to tweak the iteration a bit and should be good to go.

[omochi]

@xedin I modified to decrement loop.
As unsigned can not use x >= 0, I put exit branching at end of loop.

In this project, counter variable seems to be often unsigned.
Or should I use int as counter variable?

[omochi]

I pushed third trial.
I am testing locally now.

[xedin]

Looks good, let’s run the CI

[xedin]

@swift-ci please smoke test

[xedin]

@swift-ci please test source compatibility

[xedin]

SwiftNIO failures are unrelated, let get this merged. Thank you, again!

[omochi]

Thanks!!