Fix #1891: Don't add redundant constraint

Before adding a constraint, make sure there is no way
the two types are already in a subtype relation.

Adding redundant constraints is problematic because we
might introduce cycles. See i1891.scala for a test.

1 files changed, 1 insertions, 0 deletions

diff --git a/compiler/src/dotty/tools/dotc/core/ConstraintHandling.scala b/compiler/src/dotty/tools/dotc/core/ConstraintHandling.scala
index 0e155b9e1..42df53fed 100644
--- a/compiler/src/dotty/tools/dotc/core/ConstraintHandling.scala
+++ b/compiler/src/dotty/tools/dotc/core/ConstraintHandling.scala
@@ -57,6 +57,7 @@ trait ConstraintHandling {
           b match {
             case b: AndOrType => occursIn(b.tp1) || occursIn(b.tp2)
             case b: TypeVar => occursIn(b.origin)
+            case b: TermRef => occursIn(b.underlying)
             case _ => false
           }
         }