Fix #4272: make OrderingConstraint.&.mergeEntries symmetric

Debugging revealed that in the failing testcase `e2: TypeBounds` while `e1:
TypeRef`, and it's not clear why it should be otherwise.

Other fixes are possible, this one is easy to maintain but will repeat a few
checks.

Author: Blaisorblade

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

@@ -519,7 +519,8 @@ class OrderingConstraint(private val boundsMap: ParamBounds,
     def mergeParams(ps1: List[TypeParamRef], ps2: List[TypeParamRef]) =
       (ps1 /: ps2)((ps1, p2) => if (ps1.contains(p2)) ps1 else p2 :: ps1)
 
-    def mergeEntries(e1: Type, e2: Type): Type = e1 match {
+    @tailrec
+    def mergeEntries(e1: Type, e2: Type, retry: Boolean = true): Type = e1 match {
         case e1: TypeBounds =>
           e2 match {
             case e2: TypeBounds => e1 & e2
@@ -532,14 +533,18 @@ class OrderingConstraint(private val boundsMap: ParamBounds,
             case _ => mergeError
           }
         case _ if e1 eq e2 => e1
-        case _ => mergeError
+        case _ if retry =>
+          // mergeEntries must be commutative
+          mergeEntries(e2, e1, false)
+        case _ =>
+          mergeError
     }
 
     def mergeError = throw new AssertionError(i"cannot merge $this with $other")
 
     val that = other.asInstanceOf[OrderingConstraint]
     new OrderingConstraint(
-        merge(this.boundsMap, that.boundsMap, mergeEntries),
+        merge(this.boundsMap, that.boundsMap, mergeEntries(_, _)),
         merge(this.lowerMap, that.lowerMap, mergeParams),
         merge(this.upperMap, that.upperMap, mergeParams))
   }

tests/neg/i4272.scala

@@ -0,0 +1,4 @@
+case class C(m: Map[String, Boolean])
+object Main {
+  println(C(Map('a' -> true)))  // error
+}