Merge pull request #220 from scala/backport-lts-3.3-22479

Backport "Fix stack overflow errors when generating opaque type proxies" to 3.3 LTS

Author: tgodzik

compiler/src/dotty/tools/dotc/inlines/Inliner.scala

@@ -384,6 +384,9 @@ class Inliner(val call: tpd.Tree)(using Context):
    */
   private val opaqueProxies = new mutable.ListBuffer[(TermRef, TermRef)]
 
+  /** TermRefs for which we already started synthesising proxies */
+  private val visitedTermRefs = new mutable.HashSet[TermRef]
+
   protected def hasOpaqueProxies = opaqueProxies.nonEmpty
 
   /** Map first halves of opaqueProxies pairs to second halves, using =:= as equality */
@@ -401,8 +404,9 @@ class Inliner(val call: tpd.Tree)(using Context):
         for cls <- ref.widen.baseClasses do
           if cls.containsOpaques
              && (forThisProxy || inlinedMethod.isContainedIn(cls))
-             && mapRef(ref).isEmpty
+             && !visitedTermRefs.contains(ref)
           then
+            visitedTermRefs += ref
             val refiningRef = OpaqueProxy(ref, cls, call.span)
             val refiningSym = refiningRef.symbol.asTerm
             val refinedType = refiningRef.info

tests/pos/i22468.scala

@@ -0,0 +1,10 @@
+import Result.*
+opaque type Result[+E, +A] = Success[A] | Error[E]
+
+object Result:
+  opaque type Success[+A] = A
+  sealed abstract class Error[+E]
+
+  extension [E, A](self: Result[E, A])
+    inline def transform[B]: B = ???
+    def problem: Boolean = transform[Boolean]