Retry constraint.replace after constraint.updateEntry

Checking `isSub` on the resulting bounds may have introduced new constraints,
which might allow us to replace the type parameter entirely.

Author: EugeneFlesselle

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

@@ -302,14 +302,19 @@ trait ConstraintHandling {
       false
     else
       val bound = legalBound(param, rawBound, isUpper)
+
+      // If the narrowed bounds are equal and not recursive,
+      // we can remove `param` from the constraint.
+      def replaceIfEqualBounds: Boolean =
+        val TypeBounds(lo, hi) = constraint.nonParamBounds(param)
+        val equalBounds = (if isUpper then lo else hi) eq bound
+        if equalBounds && !bound.existsPart(_ eq param, StopAt.Static) then
+          constraint = constraint.replace(param, bound)
+          true
+        else false
+
       val oldBounds @ TypeBounds(lo, hi) = constraint.nonParamBounds(param)
-      val equalBounds = (if isUpper then lo else hi) eq bound
-      if equalBounds && !bound.existsPart(_ eq param, StopAt.Static) then
-        // The narrowed bounds are equal and not recursive,
-        // so we can remove `param` from the constraint.
-        constraint = constraint.replace(param, bound)
-        true
-      else
+      replaceIfEqualBounds || {
         // Narrow one of the bounds of type parameter `param`
         // If `isUpper` is true, ensure that `param <: `bound`, otherwise ensure
         // that `param >: bound`.
@@ -328,8 +333,13 @@ trait ConstraintHandling {
         || {
           constraint = c1
           val TypeBounds(lo, hi) = constraint.entry(param): @unchecked
-          isSub(lo, hi)
+          isSub(lo, hi) && {
+            // isSub may have introduced new constraints
+            replaceIfEqualBounds
+            true
+          }
         }
+      }
   end addOneBound
 
   protected def addBoundTransitively(param: TypeParamRef, rawBound: Type, isUpper: Boolean)(using Context): Boolean =