Fix #2663: More refined handling of enum case apply results

[odersky]

i2663 demonstrates that we do not always have a complete result type
for the apply method of an enum case. In that case our only recourse
is to add a generic widening method that upcasts the enum case class to the
enum base class.

[liufengyun]

It doesn't work for the following case:

enum Foo[T](x: T) {
  case Bar[S, T](y: T) extends Foo(y)
}

While the following compiles without no problem:

class Foo[T](x: T)
case class Bar[S, T](y: T) extends Foo(y)

[odersky]

@liufengyun Excellent catch! This turned out to be an even bigger can of worms than I thought.

[liufengyun]

The fix looks good, but I found another related issue about the return type of apply:

enum Foo[T](x: T) {
  case Bar[S, T](y: T) extends Foo[y.type](y)
}

Compiler error:

2 |  case Bar[S, T](y: T) extends Foo[2](y)
  |                                      ^
  |                                      found:    T(y)
  |                                      required: Int(2)

[odersky]

That's devious (or clever, depending how you look at it)! It seems to fix this properly we need #3920 to be solved, and it is uncertain whether we will ever get there.

The only quick fix I see is to disallow parameter references in extension clauses of enum cases.

[odersky]

Or we leave it as is, and just accept the fact that the enum expansion is not type correct in some cases.

[odersky]

I filed a separate issue #3935 to keep track of this.