Enable multi existential type variable can be override

[chengpohi]

This issue is from How to override value when value types have a different number of type parameters?
, I have tried to investigate issue and found: when compare ExistentialType with this and that has assert that their params length must be same by sameLength, but as the OP's question, there maybe has different length existential type params and their type still is legal be override.

So maybe when meet sameLength not equal, we need to try lastTry again to extract the underlying type again and check whether matches.

I have tried create a PR to fix this, Could you help review?

[SethTisue]

PR was enclosed for inactivity, but Adriaan made some encouraging comments on it

[SethTisue]

(I didn't check if this might actually be fixed; if it is fixed, let me know what version and I'll set the milestone.)

[som-snytt]

Compiles under dotty -source:future after "no overriding val parameters" rule.

class Foo[T]

class DerivedFoo[T] extends Foo[T]

class Bar(val foo: Foo[?])

class DerivedBar(override val foo: DerivedFoo[?]) extends Bar(foo)

class OtherDerivedFoo[T, U] extends Foo[T]

//when with multiple existential type, also should compile with override
class OtherDerivedBar(override val foo: OtherDerivedFoo[?,  ?]) extends Bar(foo)

or

class Foo[T]

class DerivedFoo[T] extends Foo[T]

class Bar(f00: Foo[?]) {
  val foo = f00
}

class DerivedBar(f00: DerivedFoo[?]) extends Bar(f00) {
  override val foo: DerivedFoo[?] = f00
}

class OtherDerivedFoo[T, U] extends Foo[T]

//when with multiple existential type, also should compile with override
class OtherDerivedBar(f00: OtherDerivedFoo[?,  ?]) extends Bar(f00) {
  override val foo: OtherDerivedFoo[?,  ?] = f00
}

scala/scala2 says error: value foo overrides nothing.