Try another variant

Author: odersky

Diff for: compiler/src/dotty/tools/dotc/core/TypeComparer.scala

@@ -1077,7 +1077,7 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
                         // constraint after isSubArgs, but we now pick the constraint after the dealiased test.
                         // There's one test in shapeless/deriving that had to be disabled
                         // (modules/deriving/src/test/scala/shapeless3/deriving/deriving.scala, value v7 in the functor test).
-                        sufficientEither(
+                        sufficientEither2(
                           isSubArgs(args1, args2, tp1, tparams),
                           recur(tp1.superType, tp2.superType))
                       else
@@ -1606,6 +1606,20 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
       true
     else op2
 
+  private def sufficientEither2(op1: => Boolean, op2: => Boolean): Boolean =
+    val preConstraint = constraint
+    if op1 then
+      if constraint ne preConstraint then
+        // check whether `op2` generates a weaker constraint than `op1`
+        val leftConstraint = constraint
+        constraint = preConstraint
+        if !(op2 && subsumes(leftConstraint, constraint, preConstraint)) then
+          if constr != noPrinter && !subsumes(constraint, leftConstraint, preConstraint) then
+            constr.println(i"CUT - prefer $leftConstraint over $constraint")
+          constraint = leftConstraint
+      true
+    else false
+
   /** Returns true iff the result of evaluating either `op1` or `op2` is true, keeping the smaller constraint if any.
    *  E.g., if
    *