[clang] [Frontend][Sema] Add CC1-only -fms-anonymous-structs to enable Microsoft anonymous struct/union feature (PR #176551)
Riyaz Ahmad via cfe-commits
cfe-commits at lists.llvm.org
Sun Jan 18 01:08:59 PST 2026
https://github.com/riyaz86a updated https://github.com/llvm/llvm-project/pull/176551
>From 13e881b2949f986dbd3cf82f8358c4aff0737f57 Mon Sep 17 00:00:00 2001
From: Riyaz Ahmad <riyaz.ahmad at ibm.com>
Date: Sat, 17 Jan 2026 04:16:28 -0500
Subject: [PATCH] [Frontend][Sema] Add CC1-only -fms-anonymous-structs to
enable Microsoft anonymous struct/union feature
Add a CC1-only option `-fms-anonymous-structs` to enable Microsoft anonymous
struct/union feature without enabling full Microsoft extensions.
Motivation:
- On AIX, some Microsoft extensions (e.g _ptr32) conflict with system headers.
- Certain code bases rely on Microsoft anonymous structs/unions behavior.
This new option allows selectively enabling the anonymous struct/union features in
the front-end without triggering other Microsoft extensions features.
CC1 option can be passed via clang "-Xclang -fms-anonymous-structs".
Implementation:
- Introduced CC1-only option `-fms-anonymous-structs`.
- The option updates LangOpts::MSAnonymousStructs and Updated Sema to use this flag.
- Added frontend, driver, and Sema tests.
---
clang/include/clang/Basic/LangOptions.def | 1 +
clang/include/clang/Options/Options.td | 6 ++
clang/lib/Driver/ToolChains/Clang.cpp | 4 +-
clang/lib/Sema/SemaDecl.cpp | 2 +-
clang/test/Driver/cl-options.c | 3 +
clang/test/Driver/unknown-arg.c | 2 +
clang/test/Frontend/ms-anon-structs-args.c | 15 ++++
clang/test/Sema/MicrosoftAnonymousStructs.c | 81 +++++++++++++++++++
.../Frontend/CompilerInvocationTest.cpp | 20 +++++
9 files changed, 132 insertions(+), 2 deletions(-)
create mode 100644 clang/test/Frontend/ms-anon-structs-args.c
create mode 100644 clang/test/Sema/MicrosoftAnonymousStructs.c
diff --git a/clang/include/clang/Basic/LangOptions.def b/clang/include/clang/Basic/LangOptions.def
index 36fec24638363..4420cd942cac0 100644
--- a/clang/include/clang/Basic/LangOptions.def
+++ b/clang/include/clang/Basic/LangOptions.def
@@ -47,6 +47,7 @@ LANGOPT(MSVCCompat , 1, 0, NotCompatible, "Microsoft Visual C++ full comp
LANGOPT(Kernel , 1, 0, NotCompatible, "Kernel mode")
LANGOPT(MicrosoftExt , 1, 0, NotCompatible, "Microsoft C++ extensions")
LANGOPT(ZOSExt , 1, 0, NotCompatible, "z/OS extensions")
+LANGOPT(MSAnonymousStructs, 1, 0, NotCompatible, "Microsoft anonymous struct and union extension")
LANGOPT(AsmBlocks , 1, 0, NotCompatible, "Microsoft inline asm blocks")
LANGOPT(Borland , 1, 0, NotCompatible, "Borland extensions")
LANGOPT(CPlusPlus , 1, 0, NotCompatible, "C++")
diff --git a/clang/include/clang/Options/Options.td b/clang/include/clang/Options/Options.td
index 188739e72434a..5cdca7132e779 100644
--- a/clang/include/clang/Options/Options.td
+++ b/clang/include/clang/Options/Options.td
@@ -3327,6 +3327,12 @@ def fms_extensions : Flag<["-"], "fms-extensions">, Group<f_Group>,
Visibility<[ClangOption, CC1Option, CLOption]>,
HelpText<"Accept some non-standard constructs supported by the Microsoft compiler">,
MarshallingInfoFlag<LangOpts<"MicrosoftExt">>, ImpliedByAnyOf<[fms_compatibility.KeyPath]>;
+def fms_anonymous_structs
+ : Flag<["-"], "fms-anonymous-structs">,
+ Visibility<[CC1Option]>,
+ MarshallingInfoFlag<LangOpts<"MSAnonymousStructs">>,
+ ImpliedByAnyOf<[fms_extensions.KeyPath]>,
+ HelpText<"Enable Microsoft anonymous struct/union extension.">;
defm asm_blocks : BoolFOption<"asm-blocks",
LangOpts<"AsmBlocks">, Default<fms_extensions.KeyPath>,
PosFlag<SetTrue, [], [ClangOption, CC1Option]>,
diff --git a/clang/lib/Driver/ToolChains/Clang.cpp b/clang/lib/Driver/ToolChains/Clang.cpp
index 41ee88fd5501a..b0fd41e0a026e 100644
--- a/clang/lib/Driver/ToolChains/Clang.cpp
+++ b/clang/lib/Driver/ToolChains/Clang.cpp
@@ -7156,8 +7156,10 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
// -fms-extensions=0 is default.
if (Args.hasFlag(options::OPT_fms_extensions, options::OPT_fno_ms_extensions,
- IsWindowsMSVC || IsUEFI))
+ IsWindowsMSVC || IsUEFI)) {
CmdArgs.push_back("-fms-extensions");
+ CmdArgs.push_back("-fms-anonymous-structs");
+ }
// -fms-compatibility=0 is default.
bool IsMSVCCompat = Args.hasFlag(
diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index ae779d6830d9b..4f030c6ec649f 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -5342,7 +5342,7 @@ Decl *Sema::ParsedFreeStandingDeclSpec(Scope *S, AccessSpecifier AS,
DS.getTypeSpecType() == DeclSpec::TST_typename) {
RecordDecl *Record = Tag ? dyn_cast<RecordDecl>(Tag)
: DS.getRepAsType().get()->getAsRecordDecl();
- if (Record && getLangOpts().MicrosoftExt) {
+ if (Record && getLangOpts().MSAnonymousStructs) {
Diag(DS.getBeginLoc(), diag::ext_ms_anonymous_record)
<< Record->isUnion() << DS.getSourceRange();
return BuildMicrosoftCAnonymousStruct(S, DS, Record);
diff --git a/clang/test/Driver/cl-options.c b/clang/test/Driver/cl-options.c
index 611d0d2927c43..f4ee39d3fc7d9 100644
--- a/clang/test/Driver/cl-options.c
+++ b/clang/test/Driver/cl-options.c
@@ -350,6 +350,9 @@
// WJoined: "-cc1"
// WJoined: "-Wunused-pragmas"
+// RUN: %clang_cl -c -fms-extensions -### -- %s 2>&1 | FileCheck -check-prefix=CHECK-MS-ANON-STRUCT %s
+// CHECK-MS-ANON-STRUCT: "-fms-extensions" "-fms-anonymous-structs"
+
// We recognize -f[no-]strict-aliasing.
// RUN: %clang_cl -c -### -- %s 2>&1 | FileCheck -check-prefix=DEFAULTSTRICT %s
// DEFAULTSTRICT: "-relaxed-aliasing"
diff --git a/clang/test/Driver/unknown-arg.c b/clang/test/Driver/unknown-arg.c
index 26032a407f413..b209b1bff0c9f 100644
--- a/clang/test/Driver/unknown-arg.c
+++ b/clang/test/Driver/unknown-arg.c
@@ -25,6 +25,7 @@
// This needs to exit non-0, for configure scripts.
// RUN: not %clang /GR-
+// RUN: not %clang %s -fms-anonymous-structs -### 2>&1 | FileCheck %s --check-prefix=CL-MS-ERROR
// CHECK: error: unknown argument: '-cake-is-lie'
// CHECK: error: unknown argument: '-%0'
@@ -70,3 +71,4 @@
// O-WARN: warning: joined argument treated as '-o ffload-arch=sm_70'; did you mean '--offload-arch=sm_70'?
// O-WARN-NEXT: warning: joined argument treated as '-o ffload-device-only'; did you mean '--offload-device-only'?
// O-WARN-NEXT: warning: joined argument treated as '-o utput'; did you mean '--output'?
+// CL-MS-ERROR: error: unknown argument '-fms-anonymous-structs'; did you mean '-Xclang -fms-anonymous-structs'?
diff --git a/clang/test/Frontend/ms-anon-structs-args.c b/clang/test/Frontend/ms-anon-structs-args.c
new file mode 100644
index 0000000000000..ddd3cea598959
--- /dev/null
+++ b/clang/test/Frontend/ms-anon-structs-args.c
@@ -0,0 +1,15 @@
+// Test that -fms-anonymous-structs is a CC1-only option and properly rejected by driver
+
+// RUN: %clang_cc1 -triple powerpc-ibm-aix -fms-anonymous-structs %s -fsyntax-only 2>&1 | \
+// RUN: FileCheck --check-prefix=CC1-OK %s --allow-empty
+// CC1-OK-NOT: error: unknown argument
+
+// Test that multiple occurrences are handled
+// RUN: %clang_cc1 -triple powerpc-ibm-aix -fms-anonymous-structs -fms-anonymous-structs %s -fsyntax-only 2>&1 | \
+// RUN: FileCheck --check-prefix=MULTI-OK %s --allow-empty
+// MULTI-OK-NOT: error: unknown argument
+
+// Test with other MS-related options
+// RUN: %clang_cc1 -triple powerpc-ibm-aix -fms-extensions -fms-anonymous-structs %s -fsyntax-only 2>&1 | \
+// RUN: FileCheck --check-prefix=WITH-MS-EXT %s --allow-empty
+// WITH-MS-EXT-NOT: error: unknown argument
diff --git a/clang/test/Sema/MicrosoftAnonymousStructs.c b/clang/test/Sema/MicrosoftAnonymousStructs.c
new file mode 100644
index 0000000000000..163c2f726cb67
--- /dev/null
+++ b/clang/test/Sema/MicrosoftAnonymousStructs.c
@@ -0,0 +1,81 @@
+// RUN: %clang_cc1 -triple i686-windows %s -fsyntax-only -Wno-unused-value -Wno-pointer-to-int-cast -Wmicrosoft -verify -fms-anonymous-structs
+// RUN: %clang_cc1 -triple powerpc-ibm-aix %s -fsyntax-only -Wno-unused-value -Wno-pointer-to-int-cast -Wmicrosoft -verify -fms-anonymous-structs
+
+typedef struct notnested {
+ long bad1;
+ long bad2;
+} NOTNESTED;
+
+
+typedef struct nested1 {
+ long a;
+ struct notnested var1;
+ NOTNESTED var2;
+} NESTED1;
+
+struct nested2 {
+ long b;
+ NESTED1; // expected-warning {{anonymous structs are a Microsoft extension}}
+};
+
+struct nested2 PR20573 = { .a = 3 };
+
+struct nested3 {
+ long d;
+ struct nested4 { // expected-warning {{anonymous structs are a Microsoft extension}}
+ long e;
+ };
+ union nested5 { // expected-warning {{anonymous unions are a Microsoft extension}}
+ long f;
+ };
+};
+
+typedef union nested6 {
+ long f;
+} NESTED6;
+
+struct test {
+ int c;
+ struct nested2; // expected-warning {{anonymous structs are a Microsoft extension}}
+ NESTED6; // expected-warning {{anonymous unions are a Microsoft extension}}
+};
+
+void foo(void)
+{
+ struct test var;
+ var.a;
+ var.b;
+ var.c;
+ var.bad1; // expected-error {{no member named 'bad1' in 'struct test'}}
+ var.bad2; // expected-error {{no member named 'bad2' in 'struct test'}}
+}
+
+// Enumeration types with a fixed underlying type.
+const int seventeen = 17;
+typedef int Int;
+
+void pointer_to_integral_type_conv(char* ptr) {
+ char ch = (char)ptr;
+ short sh = (short)ptr;
+ ch = (char)ptr;
+ sh = (short)ptr;
+
+ // This is valid ISO C.
+ _Bool b = (_Bool)ptr;
+}
+
+typedef struct {
+ UNKNOWN u; // expected-error {{unknown type name 'UNKNOWN'}}
+} AA;
+
+typedef struct {
+ AA; // expected-warning {{anonymous structs are a Microsoft extension}}
+} BB;
+
+struct anon_fault {
+ struct undefined; // expected-warning {{anonymous structs are a Microsoft extension}}
+ // expected-error at -1 {{field has incomplete type 'struct undefined'}}
+ // expected-note at -2 {{forward declaration of 'struct undefined'}}
+};
+
+const int anon_falt_size = sizeof(struct anon_fault);
diff --git a/clang/unittests/Frontend/CompilerInvocationTest.cpp b/clang/unittests/Frontend/CompilerInvocationTest.cpp
index 64d0f4c38f11c..cf758c4ae2c50 100644
--- a/clang/unittests/Frontend/CompilerInvocationTest.cpp
+++ b/clang/unittests/Frontend/CompilerInvocationTest.cpp
@@ -190,6 +190,26 @@ TEST_F(CommandLineTest, BoolOptionDefaultTrueSingleFlagPresent) {
ASSERT_THAT(GeneratedArgs, Contains(StrEq("-fno-temp-file")));
}
+TEST_F(CommandLineTest, MSAnonymousStructsFlagPresent) {
+ const char *Args[] = {"-cc1", "-fms-anonymous-structs"};
+
+ ASSERT_TRUE(CompilerInvocation::CreateFromArgs(Invocation, Args, *Diags));
+
+ EXPECT_TRUE(Invocation.getLangOpts().MSAnonymousStructs);
+
+ Invocation.generateCC1CommandLine(GeneratedArgs, *this);
+
+ ASSERT_THAT(GeneratedArgs, Contains(StrEq("-fms-anonymous-structs")));
+}
+
+TEST_F(CommandLineTest, MSAnonymousStructsEnabledByMSExtensions) {
+ const char *Args[] = {"clang", "-fms-extensions"};
+
+ ASSERT_TRUE(CompilerInvocation::CreateFromArgs(Invocation, Args, *Diags));
+
+ EXPECT_TRUE(Invocation.getLangOpts().MSAnonymousStructs);
+}
+
TEST_F(CommandLineTest, CC1FlagPresentWhenDoingRoundTrip) {
const char *Args[] = {"-cc1", "-round-trip-args"};
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