Fix override containing type param in explicit nulls

[noti0na1]

If a member signature contains type parameters when overriding a Java class, the check will still fail even if we retract SafeNulls from the ctx, because T | Null is not a subtype of T.

This fix will use a more relaxed approach by forcingNull being a bottom type during overriding check.

A type varibale T from Java is translated to T >: Nothing <: Any. However, null can always be a value of T for Java side. So the best solution here is to let Null be a subtype of non-primitive value types temporarily.

Example:

class D1[T <: AnyRef] extends Comparator[T]:
  override def compare(o1: T, o2: T): Int = 0

class D2[T <: AnyRef] extends Comparator[T]:
  override def compare(o1: T | Null, o2: T | Null): Int = 0

[odersky]

I find it not very appealing to run a deep type map on types for a performance critical method like matches. Is there no better/faster way to achieve what we want? Maybe a change to matches in TypeComparer?

[michelou]

In source file dotc/core/Mode.scala (line 128) I should read

val RelaxedOverriding: Mode = newMode(30, "RelaxedOverriding")

instead of

val RelaxedOverriding: Mode = newMode(30, "ForceInline")

[odersky]

Otherwise LGTM!