Rollback on failure to narrow GADT bounds

All credit to Linyxus!

Author: dwijnand

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

@@ -1945,8 +1945,11 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
       val tparam = tr.symbol
       gadts.println(i"narrow gadt bound of $tparam: ${tparam.info} from ${if (isUpper) "above" else "below"} to $bound ${bound.toString} ${bound.isRef(tparam)}")
       if (bound.isRef(tparam)) false
-      else if (isUpper) gadtAddUpperBound(tparam, bound)
-      else gadtAddLowerBound(tparam, bound)
+      else
+        val savedGadt = ctx.gadt.fresh
+        val success = if isUpper then gadtAddUpperBound(tparam, bound) else gadtAddLowerBound(tparam, bound)
+        if !success then ctx.gadt.restore(savedGadt)
+        success
     }
   }
 

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

@@ -712,16 +712,7 @@ object TypeOps:
 
     val childTp = if (child.isTerm) child.termRef else child.typeRef
 
-    val ctx1 = ctx.fresh.setExploreTyperState().setFreshGADTBounds
-    val ctx2 = parent match
-      case _: RefinedType =>
-        ctx1
-        // patmat/t9657
-        // When running Bicycle.type <:< Vehicle { A = P }
-        // TypeComparer is happy to infer GADT bounds P >: Pedal.type <: Petrol.type & Pedal.type
-        // Despite the fact that Bicycle is an object, and thus final, so its type A can only be Pedal.type.
-      case _              => ctx1.addMode(Mode.GadtConstraintInference)
-    inContext(ctx2) {
+    inContext(ctx.fresh.setExploreTyperState().setFreshGADTBounds.addMode(Mode.GadtConstraintInference)) {
       instantiateToSubType(childTp, parent).dealias
     }
   }