Support anonymous struct embedding via -fms-extensions

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;

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"))

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;

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,