Do not rely on subtyping to infer constraints for object patterns

Fixed soundness problems with abstract types - see test.

Author: abgruszecki

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -3137,7 +3137,7 @@ class Typer extends Namer
       case _: RefTree | _: Literal
         if !isVarPattern(tree) &&
            !(pt <:< tree.tpe) &&
-           !(tree.tpe <:< pt)(ctx.addMode(Mode.GADTflexible)) =>
+           !ctx.addMode(Mode.GADTflexible).typeComparer.constrainPatternType(tree.tpe, pt) =>
         val cmp =
           untpd.Apply(
             untpd.Select(untpd.TypedSplice(tree), nme.EQ),

tests/neg/unsound-union-object-gadt.scala

@@ -0,0 +1,20 @@
+object Test {
+  enum Foo[X] {
+    case Str extends Foo[String]
+    case Int extends Foo[Int]
+  }
+
+  trait Test {
+    type A
+
+    def foo[T](f: Foo[A] | Foo[T]): T =
+      f match { case Foo.Str =>
+        "" // error
+      }
+
+    def bar[T](f: Unit | Foo[T]): T =
+      f match { case Foo.Str =>
+        ""
+      }
+  }
+}