Fix bias in ConstraintHandling#subsumes

Previously, if c2 had instantiated a variable, it would not be iterated
over and would not be compared with c1.

Author: abgruszecki

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

@@ -345,7 +345,7 @@ trait ConstraintHandling[AbstractContext] {
     else {
       val saved = constraint
       try
-        c2.forallParams(p =>
+        pre.forallParams(p =>
           c1.contains(p) &&
           c2.upper(p).forall(c1.isLess(p, _)) &&
           isSubTypeWhenFrozen(c1.nonParamBounds(p), c2.nonParamBounds(p)))

tests/neg/overconstrained-type-variables-gadt.scala

@@ -2,16 +2,26 @@ object Test {
   trait T1[A] { def a: A }
   trait T2[B] { def b: B }
 
-  def foo[X, Y](u: T1[X] | T2[Y]): X = u match {
+  def foo[X, Y](v: T1[X] | T2[Y]): X = v match {
     case t1: T1[t] =>
       // consider: foo[Int, String](new T1[String] with T2[String] { ... })
       t1.a // error
   }
 
   class T1Int extends T1[Int] { def a = 0 }
-  def bar[X, Y](u: T1[X] | T2[Y]): X = u match {
+  def bar[X, Y](v: T1[X] | T2[Y]): X = v match {
     case t1: T1Int =>
       // similar reasoning to above applies
       t1.a // error
   }
+
+  class T1IntT2String extends T1[Int] with T2[String] {
+    def a = 0
+    def b = ""
+  }
+  def baz[X](v: T1[X] | T2[X]): Unit = v match {
+    case _: T1IntT2String =>
+      val x1: X = 0 // error
+      val x2: X = "" // error
+  }
 }