From 3ed8fb3a46c04e028c8c5f91fe4bedd74cc96112 Mon Sep 17 00:00:00 2001
From: Som Snytt <som.snytt@gmail.com>
Date: Wed, 28 May 2025 09:54:02 -0700
Subject: [PATCH] Warn trivial recursion with module prefix

---
 .../dotty/tools/dotc/transform/TailRec.scala  |  6 ++++-
 tests/warn/i23277.scala                       | 22 +++++++++++++++++++
 2 files changed, 27 insertions(+), 1 deletion(-)
 create mode 100644 tests/warn/i23277.scala

diff --git a/compiler/src/dotty/tools/dotc/transform/TailRec.scala b/compiler/src/dotty/tools/dotc/transform/TailRec.scala
index b8052721ff27..472ed78bfb6b 100644
--- a/compiler/src/dotty/tools/dotc/transform/TailRec.scala
+++ b/compiler/src/dotty/tools/dotc/transform/TailRec.scala
@@ -196,7 +196,8 @@ class TailRec extends MiniPhase {
         def isInfiniteRecCall(tree: Tree): Boolean = {
           def tailArgOrPureExpr(stat: Tree): Boolean = stat match {
             case stat: ValDef if stat.name.is(TailTempName) || !stat.symbol.is(Mutable) => tailArgOrPureExpr(stat.rhs)
-            case Assign(lhs: Ident, rhs) if lhs.symbol.name.is(TailLocalName) => tailArgOrPureExpr(rhs)
+            case Assign(lhs: Ident, rhs) if lhs.symbol.name.is(TailLocalName) =>
+              tailArgOrPureExpr(rhs) || varForRewrittenThis.exists(_ == lhs.symbol && rhs.tpe.isStable)
             case Assign(lhs: Ident, rhs: Ident) => lhs.symbol == rhs.symbol
             case stat: Ident if stat.symbol.name.is(TailLocalName) => true
             case _ => tpd.isPureExpr(stat)
@@ -345,6 +346,9 @@ class TailRec extends MiniPhase {
               case prefix: This if prefix.symbol == enclosingClass =>
                 // Avoid assigning `this = this`
                 assignParamPairs
+              case prefix if prefix.symbol.is(Module) && prefix.symbol.moduleClass == enclosingClass =>
+                // Avoid assigning `this = MyObject`
+                assignParamPairs
               case _ =>
                 (getVarForRewrittenThis(), noTailTransform(prefix)) :: assignParamPairs
 
diff --git a/tests/warn/i23277.scala b/tests/warn/i23277.scala
new file mode 100644
index 000000000000..f99f5a373d43
--- /dev/null
+++ b/tests/warn/i23277.scala
@@ -0,0 +1,22 @@
+
+enum Test:
+  case One
+  case Two(i: Int)
+
+object Test:
+  object Two:
+    def apply(i: Int): Test.Two = Test.Two(i) // warn
+    def apply(i: Int, j: Int): Test.Two = new Test.Two(i+j)
+    def apply(i: Int, s: String): Two = Two(i, s) // warn because unprefixed Two is deemed pure
+    def apply(): Test.Two = other.apply() // nowarn prefix is method call
+    def other: this.type = this
+
+object R extends Runnable:
+  def r: this.type = this
+  override def run() = r.run()
+
+final class C(c: C):
+  def f(i: Int): Int = c.f(i)
+
+@main def main = println:
+  Test.Two(1)