[clang] [clang] Accept empty enum in MSVC compatible C (PR #159981)

[via cfe-commits]

https://github.com/yicuixi updated https://github.com/llvm/llvm-project/pull/159981

>From 77fc06c7147e8eec1712f2c8babdb6fd8a48dd94 Mon Sep 17 00:00:00 2001
From: yicuixi <qin_17914 at 126.com>
Date: Sun, 21 Sep 2025 22:22:28 +0800
Subject: [PATCH 1/2] [clang] Accept empty enum in MSVC compatible C

Signed-off-by: yicuixi <qin_17914 at 126.com>
---
 clang/docs/ReleaseNotes.rst       | 1 +
 clang/lib/Parse/ParseDecl.cpp     | 3 ++-
 clang/test/Parser/ms-empty-enum.c | 8 ++++++++
 3 files changed, 11 insertions(+), 1 deletion(-)
 create mode 100644 clang/test/Parser/ms-empty-enum.c

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 46d56bb3f07f5..c4b5e15b2b4f9 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -366,6 +366,7 @@ Bug Fixes in This Version
 - Fixed an assertion when an improper use of the ``malloc`` attribute targeting
   a function without arguments caused us to try to access a non-existent argument.
   (#GH159080)
+- Accept empty enum in MSVC-compatible C. (#GH114402)
 
 Bug Fixes to Compiler Builtins
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/lib/Parse/ParseDecl.cpp b/clang/lib/Parse/ParseDecl.cpp
index 22c01c4e371f3..17e29b3efc348 100644
--- a/clang/lib/Parse/ParseDecl.cpp
+++ b/clang/lib/Parse/ParseDecl.cpp
@@ -5360,7 +5360,8 @@ void Parser::ParseEnumBody(SourceLocation StartLoc, Decl *EnumDecl,
   T.consumeOpen();
 
   // C does not allow an empty enumerator-list, C++ does [dcl.enum].
-  if (Tok.is(tok::r_brace) && !getLangOpts().CPlusPlus)
+  if (Tok.is(tok::r_brace) && !getLangOpts().CPlusPlus &&
+      !getLangOpts().MSVCCompat && !getLangOpts().MicrosoftExt)
     Diag(Tok, diag::err_empty_enum);
 
   SmallVector<Decl *, 32> EnumConstantDecls;
diff --git a/clang/test/Parser/ms-empty-enum.c b/clang/test/Parser/ms-empty-enum.c
new file mode 100644
index 0000000000000..37e34d7c154bf
--- /dev/null
+++ b/clang/test/Parser/ms-empty-enum.c
@@ -0,0 +1,8 @@
+// RUN: %clang_cc1 %s -fsyntax-only -Wmicrosoft -verify -fms-extensions
+// RUN: %clang_cc1 %s -fsyntax-only -Wmicrosoft -verify -fms-compatibility
+// expected-no-diagnostics
+
+typedef enum tag1 { } A;
+typedef enum tag2 { } B;
+typedef enum : unsigned { } C;
+

>From 191eacf08e22c443bd1b091fc97d4a3ca13d9cee Mon Sep 17 00:00:00 2001
From: yicuixi <qin_17914 at 126.com>
Date: Sun, 21 Sep 2025 23:17:18 +0800
Subject: [PATCH 2/2] Fix test

Signed-off-by: yicuixi <qin_17914 at 126.com>
---
 clang/test/Parser/ms-empty-enum.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/clang/test/Parser/ms-empty-enum.c b/clang/test/Parser/ms-empty-enum.c
index 37e34d7c154bf..7508f7fd0d0d8 100644
--- a/clang/test/Parser/ms-empty-enum.c
+++ b/clang/test/Parser/ms-empty-enum.c
@@ -1,8 +1,7 @@
 // RUN: %clang_cc1 %s -fsyntax-only -Wmicrosoft -verify -fms-extensions
 // RUN: %clang_cc1 %s -fsyntax-only -Wmicrosoft -verify -fms-compatibility
-// expected-no-diagnostics
 
 typedef enum tag1 { } A;
 typedef enum tag2 { } B;
-typedef enum : unsigned { } C;
+typedef enum : unsigned { } C; // expected-warning {{enumeration types with a fixed underlying type are a Microsoft extension}}