Fix subsumes test between constraints

The test gave a false negative in the case where a parameter was
instantiated to the same value in both constraints. In that case
the parameter is not "contained" in either constraint, since it
is already instantiated, and that caused the test to fail.

Fixes #13541

Author: odersky

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

@@ -411,9 +411,10 @@ trait ConstraintHandling {
         // If `c2` has, compared to `pre`, instantiated a param and we iterated over params of `c2`,
         // we could miss that param being instantiated to an incompatible type in `c1`.
         pre.forallParams(p =>
-          c1.contains(p) &&
-          c2.upper(p).forall(c1.isLess(p, _)) &&
-          isSubTypeWhenFrozen(c1.nonParamBounds(p), c2.nonParamBounds(p)))
+          c1.entry(p).exists
+          && c2.upper(p).forall(c1.isLess(p, _))
+          && isSubTypeWhenFrozen(c1.nonParamBounds(p), c2.nonParamBounds(p))
+        )
       finally constraint = saved
     }
 

tests/pos/i13541.scala

@@ -0,0 +1,14 @@
+trait F[A]
+trait Z
+object Z:
+  given F[Z] = ???
+
+type Foo[B] = [A] =>> Bar[A, B]
+trait Bar[A, B]
+
+given fooUnit[A: F]: Foo[Unit][A] = ???
+//given bar[A: F]: Bar[A, Unit] = ???
+
+def f[A: F](using Foo[Unit][A]): Nothing = ???
+
+def broken: Nothing = f[Z]