Feature GADT support for class type parameters

[Ang9876]

• Register GADT constraint class type parameters from outer contexts in typedDefdef.
• Remove a condition to support GADT approximation for class type parameters.
• Add test cases in tests/pos/class-gadt.
• Ignore test cases tests/pos/i4345.scala and i5735.scala (issue Superfluous asInstanceOf breaking singleton types #11220 and Assertion failure when check the subtype relation #11221)

[abgruszecki]

Hey Ang, the code looks good. Good catch with the nested classes. Could you rewrite the code to look up "outer" classes instead of accumulating class type parameters? We usually try to avoid storing information in context that we can otherwise get.

When class B is nested inside A, then the owner of B should be A. You'll need to recursively get owners which are classes.

[Ang9876]

Hey Ang, the code looks good. Good catch with the nested classes. Could you rewrite the code to look up "outer" classes instead of accumulating class type parameters? We usually try to avoid storing information in context that we can otherwise get.

When class B is nested inside A, then the owner of B should be A. You'll need to recursively get owners which are classes.

Hi Alex, thanks a lot for the review. I just changed the code and used outersInterator.
But I have a question about flags. If I use

takeWhile(!_.owner.flags.is(Method))

instead of

takeWhile(!_.owner.is(Method))

Building dotty would fail with an error

[error] ## Exception when compiling 478 sources to /.../dotty-1/compiler/../out/bootstrap/scala3-compiler-bootstrapped/scala-3.0.0-RC1/classes

What is the difference between sym.is() and sym.flags.is()?

[abgruszecki]

@Ang9876 there seem to be some optimisations in Symbol#is, perhaps you've found a problem with those optimisations. Let's check after merging your PR if the problem is still there and if so, open an issue.

[abgruszecki]

Alright, this looks good. Thanks for your work, @Ang9876 !