Fix #3246: Refine handling of postfix _ for non-functions

[allanrenucci]

Reopening #3300 from dotty-staging

[allanrenucci]

This used to compile but doesn't compile anymore:

class Test {
  class A
  def foo(x: A)(y: x.type) = ()
  val bar = foo(new A()) _
 }

-- Error: tests/allan/Test.scala:4:25 ------------------------------------------
4 |  val bar = foo(new A()) _
  |            ^^^^^^^^^^^^^^
  |            not a function: foo(new A()); cannot be followed by `_'

one error found

[allanrenucci]

Same for:

class Test {
  class A {
    type T
    def f(x: T): T = ???
  }
  def a: A = new A
  val f = a.f _
 }

-- Error: tests/allan/Test.scala:7:14 ------------------------------------------
7 |  val f = a.f _
  |          ^^^^^
  |          not a function: a.f; cannot be followed by `_'

one error found

[liufengyun]

@allanrenucci The two examples don't compile by 2.12.3, do we need to support in Dotty?

[allanrenucci]

They compile on Dotty master and are extracted from our test suite.

I believe the second example should probably compile or at least this should compile (it compiles with scalac):

class Test {
  class A {
    type T = Int
    def f(x: T): T = ???
  }
  def a: A = new A
  val f = a.f _
}

Regarding the first example, I am not sure

[liufengyun]

It works now, review @allanrenucci , thanks.

[allanrenucci]

LGTM. Thanks!