From c0a1b97a96bfde9c4ede716c9d150548cca291b3 Mon Sep 17 00:00:00 2001
From: Dan DaCosta <dandaco@amazon.com>
Date: Wed, 27 May 2026 21:11:01 +0000
Subject: [PATCH] Support anonymous struct embedding via -fms-extensions
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When -fms-extensions is passed, tagged struct/union members without a
declarator name are treated as anonymous embedding (Plan 9 C extension).
This allows code using the GCC extension where 'struct tag;' inside
another struct inlines the tagged struct's fields.

The implementation lets the tagged anonymous member fall through to the
existing anonymous member handling (the $anon naming + set_anonymous
pass), rather than skipping it with 'continue'.

Only this one behavior is implemented — no other MSVC or Plan 9
extensions. The internal config field is named
allow_anonymous_struct_embedding to be explicit about scope.
---
 src/ansi-c/c_typecheck_type.cpp | 6 ++++++
 src/goto-cc/gcc_mode.cpp        | 4 ++++
 src/util/config.cpp             | 1 +
 src/util/config.h               | 1 +
 4 files changed, 12 insertions(+)

diff --git a/src/ansi-c/c_typecheck_type.cpp b/src/ansi-c/c_typecheck_type.cpp
index 2362e3966f6..82473e35bd0 100644
--- a/src/ansi-c/c_typecheck_type.cpp
+++ b/src/ansi-c/c_typecheck_type.cpp
@@ -1008,6 +1008,12 @@ void c_typecheck_baset::typecheck_compound_body(
             {
               // ok, anonymous union
             }
+            else if(config.ansi_c.allow_anonymous_struct_embedding)
+            {
+              // With -fms-extensions, tagged struct/union members without
+              // a declarator name are treated as anonymous embedding
+              // (Plan 9 C extension)
+            }
             else
             {
               continue;
diff --git a/src/goto-cc/gcc_mode.cpp b/src/goto-cc/gcc_mode.cpp
index d48c4234abb..3ac41dd2ab1 100644
--- a/src/goto-cc/gcc_mode.cpp
+++ b/src/goto-cc/gcc_mode.cpp
@@ -579,6 +579,10 @@ int gcc_modet::doit()
     config.ansi_c.wchar_t_is_unsigned=true;
   }
 
+  // -fms-extensions: only anonymous struct embedding is implemented.
+  if(cmdline.isset("fms-extensions"))
+    config.ansi_c.allow_anonymous_struct_embedding=true;
+
   // -fsingle-precision-constant makes floating-point constants "float"
   // instead of double
   if(cmdline.isset("-fsingle-precision-constant"))
diff --git a/src/util/config.cpp b/src/util/config.cpp
index da70a3b9888..e765d72ad38 100644
--- a/src/util/config.cpp
+++ b/src/util/config.cpp
@@ -867,6 +867,7 @@ bool configt::set(const cmdlinet &cmdline)
   cpp.cpp_standard=cppt::default_cpp_standard();
 
   ansi_c.single_precision_constant=false;
+  ansi_c.allow_anonymous_struct_embedding=false;
   ansi_c.for_has_scope=true; // C99 or later
   ansi_c.ts_18661_3_Floatn_types=false;
   ansi_c.__float128_is_keyword = false;
diff --git a/src/util/config.h b/src/util/config.h
index e1cdc3eb99c..17c96d36868 100644
--- a/src/util/config.h
+++ b/src/util/config.h
@@ -168,6 +168,7 @@ class configt
     bool bf16_type;               // __bf16 (Clang >= 15, GCC >= 13)
     bool fp16_type;               // __fp16 (GCC >= 4.5 on ARM, Clang >= 6)
     bool single_precision_constant;
+    bool allow_anonymous_struct_embedding; // -fms-extensions (partial)
     enum class c_standardt
     {
       C89,