Fix #2732: Allow wildcards in SAM types

[smarter]

The isInstantiable test as written is incorrect:
Given tp = java.util.function.Function[String, _ <: String], we will have:
tref = skolem TermRef with underlying info tp
selfType = java.util.function.Function[String, tref#R] (because of the asSeenFrom)
tref <:< selfType is false, because tref.underlying is
just tp and its return type parameter is a bounded wildcard, not a reference with
tref as a prefix.
In any case, I cannot think of a case where this function should return
false, and no test failed when I removed it. So I'm removing it until
someone can come up with a counter-example.

[odersky]

Here's a test:

class B {
  def f = 1
}

trait A { self: B =>

  def g1(x: Int): Int
  def g2 = g1(f)

}

object Test extends App {

  val x: A = (y: Int) => y + y

  println(x.g2)

}

With master this gives:

-- [E007] Type Mismatch Error: ../neg/i3347.scala:14:30 ------------------------
14 |  val x: A = (y: Int) => y + y
   |                              ^
   |                              found:    Int => Int
   |                              required: A
   |        

With this PR it compiles but we get a ClassCast exception at runtime.

~/workspace/dotty/tests/pos> java Test
Exception in thread "main" java.lang.ExceptionInInitializerError
at Test.main(i3347.scala)
Caused by: java.lang.ClassCastException: Test$$$Lambda$10/425918570 cannot be cast to B
at A.g2(i3347.scala:8)
at Test$.(i3347.scala:16)
at Test$.(i3347.scala)
... 1 more

[odersky]

As the test shows, we can't drop isInstantiatable

[smarter]

Indeed. Can you think of a way to fix isInstantiable ? Maybe we could use a TypeMap to rewrite Function[String, _ <: String] to Function[String, String] before calling .narrow, but that's not very nice.

[liufengyun]

test performance please

(meta test of the bench infrastructure)

[dottybot]

performance test scheduled: 1 job(s) in queue, 0 running.

[dottybot]

performance test failed:

Error line number: 19

[check /data/workspace/bench/logs/pull-3347-10-26-18.18.out for more information]

[odersky]

Indeed. Can you think of a way to fix isInstantiable?

For the moment it's too much of a context switch for me to think of a fix.

[smarter]

Superceded by #4152