Fix #4611

[allanrenucci]

class and trait with a single abstract method which result type is an
implicit function type are not considered SAM types anymore. The reason
is because the abstract member desugar to mutiple ones. E.g.

trait Foo {
  def foo(): implicit Int => Int
}

// becomes
trait Foo {
  def foo(): implicit Int => Int
  def foo$direct(implicit x: Int): Int
}

[allanrenucci]

An alternative fix which did not introduce this restriction on SAM types was tried in #4621.

For now this fixes the compiler crash. In the future, if one wants to try again and lift the restriction, implicitFuns2.scala is a very good test case

[allanrenucci]

Blocked on #4753

[smarter]

Can you add a testcase where the implicit result type comes from a type parameter e.g.:

trait Foo[T] {
  def bla(x: Int): T
}

val a: Foo[implicit Int => Int] = (x: Int) => 1 // error

[smarter]

which -> whose