Consider TypeBounds not fully instantiated

Author: mbovel

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

@@ -2725,7 +2725,7 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
         x && {
           t.dealias match {
             case tp: TypeRef if !tp.symbol.isClass => false
-            case _: SkolemType | _: TypeVar | _: TypeParamRef => false
+            case _: SkolemType | _: TypeVar | _: TypeParamRef | _: TypeBounds => false
             case _ => foldOver(x, t)
           }
         }

tests/pos/i16869.scala

@@ -0,0 +1,13 @@
+class C[T]
+
+type Foo[T] = T match
+  case C[true] => true
+  case C[false] => false
+
+class W[T] extends C[Foo[T]]
+
+def f[T <: C[?]](t: T) = W[T]()
+
+def test =
+  val b = C[true]()
+  f(b): C[true]