Update invariantDisjoint check

The previous check was too conservative, is we have a proof that A and B are
disjoint, then A and B in Inv[A] and Inv[B] are clearly different types, and
Inv[A] and Inv[B] are disjoint. Also the isSameType() && fullyInstantiated
check is quite primitive, it can still be useful in some cases.

Author: OlivierBlanvillain

compiler/src/dotty/tools/dotc/core/TypeComparer.scala

@@ -2552,17 +2552,17 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
         def covariantDisjoint(tp1: Type, tp2: Type, tparam: TypeParamInfo): Boolean =
           provablyDisjoint(tp1, tp2) && typeparamCorrespondsToField(tycon1, tparam)
 
-        // In the invariant case, we used a weaker version of disjointness:
-        // we consider types not equal with respect to =:= to be disjoint
+        // In the invariant case, we also use a stronger notion of disjointness:
+        // we consider fully instantiated types not equal wrt =:= to be disjoint
         // (under any context). This is fine because it matches the runtime
         // semantics of pattern matching. To implement a pattern such as
         // `case Inv[T] => ...`, one needs a type tag for `T` and the compiler
         // is used at runtime to check it the scrutinee's type is =:= to `T`.
-        // Note that this is currently a theoretical concern since we Dotty
+        // Note that this is currently a theoretical concern since Dotty
         // doesn't have type tags, meaning that users cannot write patterns
         // that do type tests on higher kinded types.
         def invariantDisjoint(tp1: Type, tp2: Type, tparam: TypeParamInfo): Boolean =
-          covariantDisjoint(tp1, tp2, tparam) || !isSameType(tp1, tp2) && {
+          provablyDisjoint(tp1, tp2) || !isSameType(tp1, tp2) && {
             // We can only trust a "no" from `isSameType` when both
             // `tp1` and `tp2` are fully instantiated.
             def fullyInstantiated(tp: Type): Boolean = new TypeAccumulator[Boolean] {