From 6d9b0f14f18b89d1719fd5346284eae9cc6e39bc Mon Sep 17 00:00:00 2001
From: Jan Chyb <48855024+jchyb@users.noreply.github.com>
Date: Thu, 30 Jan 2025 11:53:37 +0100
Subject: [PATCH] Fix stack overflow errors when generating opaque type proxies
 (#22479)

Before the regressive PR, we would check via the generated opaqueProxies
list whether one was already generated. In that PR, we tried allowing
generating proxies for rhs of currently generated proxy. Since we have
to add the generated proxy to opaqueProxies only after that step, this
could cause infinite recursion (and adding the proxies earlier could
cause another infinite loop).

To fix that, we add another collection for termrefs which we already
visited this way, but which is not used in the `mapOpaques` function.
---
 compiler/src/dotty/tools/dotc/inlines/Inliner.scala |  6 +++++-
 tests/pos/i22468.scala                              | 10 ++++++++++
 2 files changed, 15 insertions(+), 1 deletion(-)
 create mode 100644 tests/pos/i22468.scala

diff --git a/compiler/src/dotty/tools/dotc/inlines/Inliner.scala b/compiler/src/dotty/tools/dotc/inlines/Inliner.scala
index 982d15f8bcf7..27d95d055f40 100644
--- a/compiler/src/dotty/tools/dotc/inlines/Inliner.scala
+++ b/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
diff --git a/tests/pos/i22468.scala b/tests/pos/i22468.scala
new file mode 100644
index 000000000000..265d6d3a89b7
--- /dev/null
+++ b/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]