From f50529f4f607acfda6205f109af4810737d33e42 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Oliver=20Bra=C4=8Devac?= <bracevac@users.noreply.github.com>
Date: Sun, 5 Oct 2025 20:38:15 +0200
Subject: [PATCH] Fix cyclicity error for self-referential enum case

Fixes #11443

When an enum case references itself in a parent type (e.g.,
`case Nn extends Opt[Nothing] with Comparable[Nn.type]`), the
compiler previously reported a cyclic reference error during type
inference.

The fix attempts to provide an explicit type annotation
(the intersection of all parents) for parameterless enum cases
early at desugaring time.
---
 compiler/src/dotty/tools/dotc/ast/DesugarEnums.scala | 10 +++++++++-
 tests/neg/i6601.scala                                | 10 +++++-----
 tests/pos/i11443.scala                               |  5 +++++
 tests/{warn => pos}/i21860.scala                     |  2 +-
 4 files changed, 20 insertions(+), 7 deletions(-)
 create mode 100644 tests/pos/i11443.scala
 rename tests/{warn => pos}/i21860.scala (77%)

diff --git a/compiler/src/dotty/tools/dotc/ast/DesugarEnums.scala b/compiler/src/dotty/tools/dotc/ast/DesugarEnums.scala
index 7268ec720ce2..abe0abf05ede 100644
--- a/compiler/src/dotty/tools/dotc/ast/DesugarEnums.scala
+++ b/compiler/src/dotty/tools/dotc/ast/DesugarEnums.scala
@@ -289,7 +289,15 @@ object DesugarEnums {
       val (tag, scaffolding) = nextOrdinal(name, CaseKind.Object, definesLookups)
       val impl1 = cpy.Template(impl)(parents = impl.parents :+ scalaRuntimeDot(tpnme.EnumValue), body = Nil)
         .withAttachment(ExtendsSingletonMirror, ())
-      val vdef = ValDef(name, TypeTree(), New(impl1)).withMods(mods.withAddedFlags(EnumValue, span))
+
+      /** i11443: Attempt to provide an explicit type annotation for the enum case to allow certain cycles.
+       *  We pick the intersection of all parents, but only if they can be determined to be all types at this point.
+       *  Notably, this doesn't hold if one of the parents is a constructor call (e.g., extends Planet("Pluto")),
+       *  which might involve yet-to-be inferred generic parameters.
+       */
+      val tpt = if impl.parents.forall(_.isType) then impl.parents.reduceLeft(makeAndType(_, _)) else TypeTree()
+
+      val vdef = ValDef(name, tpt, New(impl1)).withMods(mods.withAddedFlags(EnumValue, span))
       flatTree(vdef :: scaffolding).withSpan(span)
     }
   }
diff --git a/tests/neg/i6601.scala b/tests/neg/i6601.scala
index 69c508ddc262..b6ab0a6c22d2 100644
--- a/tests/neg/i6601.scala
+++ b/tests/neg/i6601.scala
@@ -1,22 +1,22 @@
 object GADTs2 {
   enum Var[G, A] {
     case Z[A, G]() extends Expr[(A, G), A] // error
-    case X extends AnyRef                  // error
+    case X extends AnyRef                  // error // error
   }
   enum Expr[G, A] {
     case Lit[G](n: Int) extends Expr[G, Int]
         // case S[A, G](x:
   }
   enum Covariant[+T] {
-    case Bottom extends AnyRef // error
+    case Bottom extends AnyRef // error // error
   }
   enum Contravariant[-T] {
-    case Top extends AnyRef // error
+    case Top extends AnyRef // error // error
   }
   enum Color {
-    case Red extends AnyRef // error
+    case Red extends AnyRef // error // error
   }
   enum Parameterized[T](x: T) {
-    case Foo extends AnyRef // error
+    case Foo extends AnyRef // error // error
   }
 }
diff --git a/tests/pos/i11443.scala b/tests/pos/i11443.scala
new file mode 100644
index 000000000000..b1a995ad80c8
--- /dev/null
+++ b/tests/pos/i11443.scala
@@ -0,0 +1,5 @@
+enum Opt[T] {
+  case Nn extends Opt[Nothing] with Comparable[Nn.type]
+
+  def compareTo(nn: Nn.type) = 0
+}
\ No newline at end of file
diff --git a/tests/warn/i21860.scala b/tests/pos/i21860.scala
similarity index 77%
rename from tests/warn/i21860.scala
rename to tests/pos/i21860.scala
index 377d4761e80f..c3e16d9d443d 100644
--- a/tests/warn/i21860.scala
+++ b/tests/pos/i21860.scala
@@ -8,7 +8,7 @@ enum Shape extends Figure:
   case Ellipsis extends Shape
 
 def hasCorners(s: Shape): Boolean = s match
-  case hasCorners: Corners => true //  <--- reported as `Unreachable case`
+  case hasCorners: Corners => true //  <--- previously reported as `Unreachable case` (i21860), ok now (i11443)
   case _                   => false
 
 class Test: