From a0ecdbfbfe57a57ab33bdb5e1de4d5dfa8407dbb Mon Sep 17 00:00:00 2001
From: Aaron Patterson <tenderlove@ruby-lang.org>
Date: Tue, 15 Oct 2024 16:57:09 -0700
Subject: [PATCH] Remove "simple" flag from forwarded ICs

I don't think we should ever consider forwarded IC's to be "simple".
Previously, the "simple" flag would be copied to the derived IC and this
happened to cause struct set / get iseqs to write an invalid CC
fastpath:

  https://github.com/tenderlove/ruby/blob/f45eb3dcb9c7d849064cb802953f37e1cf9f3996/vm_insnhelper.c#L4726-L4729

[Bug #20799]
---
 bootstraptest/test_method.rb | 21 +++++++++++++++++++++
 vm_args.c                    |  2 +-
 2 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/bootstraptest/test_method.rb b/bootstraptest/test_method.rb
index af9443b8c6..f6f04541d6 100644
--- a/bootstraptest/test_method.rb
+++ b/bootstraptest/test_method.rb
@@ -1374,3 +1374,24 @@ assert_equal 'ok', %q{
   foo(:foo, b: :ok)
   foo(*["foo"], b: :ok)
 }
+
+assert_equal 'ok', %q{
+  Thing = Struct.new(:value)
+
+  Obj = Thing.new("ok")
+
+  def delegate(...)
+    Obj.value(...)
+  end
+
+  def no_args
+    delegate
+  end
+
+  def splat_args(*args)
+    delegate(*args)
+  end
+
+  no_args
+  splat_args
+}
diff --git a/vm_args.c b/vm_args.c
index 0ecdc31a31..28f685d7dd 100644
--- a/vm_args.c
+++ b/vm_args.c
@@ -1177,7 +1177,7 @@ vm_caller_setup_fwd_args(const rb_execution_context_t *ec, rb_control_frame_t *r
 
     *adjusted_ci = VM_CI_ON_STACK(
             site_mid,
-            (caller_flag | (site_flag & (VM_CALL_FCALL | VM_CALL_FORWARDING))),
+            ((caller_flag & ~VM_CALL_ARGS_SIMPLE) | (site_flag & (VM_CALL_FCALL | VM_CALL_FORWARDING))),
             site_argc + caller_argc,
             kw
             );