Error message with capture sets variables can be confusing

[odersky]

Compiler version

3.6.2

Minimized example

import language.experimental.captureChecking

class Cap extends caps.Capability

def test(cap1: Cap, cap2: Cap) =
  def f2(): Int ->{cap1} Int = ???
  def h(ff: () -> Int ->{cap2} Int) = ???
  h(() => f2())  // error

Output

-- [E007] Type Mismatch Error: ../../new/test.scala:8:12 -----------------------
8 |  h(() => f2())  // error
  |          ^^^^
  |          Found:    Int ->{cap1} Int
  |          Required: (x$0: Int) ->? Int

Expectation

I would have liked to see instead:

  |          Required: (x$0: Int) ->{cap2} Int

The problem is that there a capture set variable generated for the closure result which has {cap2} as its upper capture set. That information is important, but gets suppressed in the error diagnostic. We could take all contravariant capture sets in the expected type and interpolate each of them with the maximal set that satisfies their constraints. Then, if the subtype test still fails we could use the interpolated sets instead of the original variables. That would help.