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.

Author: odersky

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
           }
         }

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

@@ -375,11 +375,19 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
       thirdTryNamed(tp1, tp2)
     case tp2: PolyParam =>
       def comparePolyParam =
-        (ctx.mode is Mode.TypevarsMissContext) ||
-        isSubTypeWhenFrozen(tp1, bounds(tp2).lo) || {
+        (ctx.mode is Mode.TypevarsMissContext) || {
+        val alwaysTrue =
+          // The following condition is carefully formulated to catch all cases
+          // where the subtype relation is true without needing to add a constraint
+          // It's tricky because we might need to either appriximate tp2 by its
+          // lower bound or else widen tp1 and check that the result is a subtype of tp2.
+          if (frozenConstraint || alwaysFluid) isSubType(tp1, bounds(tp2).lo)
+          else isSubTypeWhenFrozen(tp1, tp2)
+        alwaysTrue || {
           if (canConstrain(tp2)) addConstraint(tp2, tp1.widenExpr, fromBelow = true)
           else fourthTry(tp1, tp2)
         }
+      }
       comparePolyParam
     case tp2: RefinedType =>
       def compareRefinedSlow: Boolean = {

tests/pos/i1891.scala

@@ -0,0 +1,11 @@
+object Test {
+  class CC2[A, B](a: A, b: B)
+
+  type T2[A, B] = CC2[A, B]
+
+  class ArrowAssoc[A](val self: A) {
+    @inline def f[B](y: B): CC2[A, B] = new CC2(self, y)
+  }
+
+  def foo = (new ArrowAssoc(1)).f(2)
+}