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

yicuixi · 2025-09-21T14:24:45Z

Fixes #114402.
This patch accept empty enum in C as a microsoft extension and introduce an new warning -Wmicrosoft-empty-enum.

Signed-off-by: yicuixi <[email protected]>

llvmbot · 2025-09-21T14:25:19Z

@llvm/pr-subscribers-clang

Author: None (yicuixi)

Changes

Fixes #114402.

Full diff: https://github.com/llvm/llvm-project/pull/159981.diff

3 Files Affected:

(modified) clang/docs/ReleaseNotes.rst (+1)
(modified) clang/lib/Parse/ParseDecl.cpp (+2-1)
(added) clang/test/Parser/ms-empty-enum.c (+8)

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;
+

Signed-off-by: yicuixi <[email protected]>

yronglin

Thanks, LGTM!

clang/lib/Parse/ParseDecl.cpp

erichkeane

IF we're accepting this as an extension, we need to emit a diagnostic to that effect. See the Ext-Warn already with an underlying type (which we already supported), we need to do something similar for these.

shafik

Can you please explain the problem in the summary instead of just pointing to an issue. The summary should explain basics required to understand the PR w/o having to jump to the issue right away for context.

So, What are you changing, why are you changing it and how.

yicuixi · 2025-09-24T17:50:09Z

Can you please explain the problem in the summary instead of just pointing to an issue. The summary should explain basics required to understand the PR w/o having to jump to the issue right away for context.

So, What are you changing, why are you changing it and how.

Done

github-actions · 2025-09-24T17:51:56Z

✅ With the latest revision this PR passed the C/C++ code formatter.

yronglin · 2025-09-29T15:42:33Z

Ping @erichkeane

clang/docs/ReleaseNotes.rst

erichkeane · 2025-09-29T16:01:12Z

clang/test/Parser/ms-empty-enum.c

+
+typedef enum tag1 { } A; // expected-warning {{empty enumeration types are a Microsoft extension}}
+typedef enum tag2 { } B; // expected-warning {{empty enumeration types are a Microsoft extension}}
+typedef enum : unsigned { } C; // expected-warning {{enumeration types with a fixed underlying type are a Microsoft extension}}\


This shouldn't warn 2x, the 'with a fixed underlying type' version is sufficient.

Also, isn't this a C23 extension, not a microsoft extension (or at least, as well)? See: ext_c23_enum_fixed_underlying_type.

Thanks for the review!

Also, isn't this a C23 extension, not a microsoft extension (or at least, as well)? See: ext_c23_enum_fixed_underlying_type.

In microsoft-extension mode, it's ext_ms_c_enum_fixed_underlying_type but not ext_ms_c_enum_fixed_underlying_type.

This shouldn't warn 2x, the 'with a fixed underlying type' version is sufficient.

Should we suppress the 2nd empty enumeration types are a Microsoft extension warning? Seems they are 2 different issue.

I'm a little confused here as to the order of warnings/what we should do. I suspect @AaronBallman and I need to spend some time figuring out/discussing what we're looking for.

I'm not sure about the suppressing the 2nd warning, there is perhaps a 'we should do both' kinda thing since one might be disabled, but it also looks silly.

Ping this in another ~week and a half, and Aaron and I can discuss it (he should be back by then!).

I think emitting two diagnostics is fine (one for the underlying type and one for the empty enum) because they are separate extensions. But I think it's a bit odd that we claim the underlying type is a Microsoft extension rather than a C23 extension -- that seems to be a preexisting issue which could be handled in a separate PR: https://godbolt.org/z/sMhGoE3qq

address comments Co-authored-by: Erich Keane <[email protected]>

yicuixi · 2025-10-07T13:22:38Z

@erichkeane friendly ping

erichkeane

@AaronBallman is back now! He will hopefully be getting to this soon.

AaronBallman

Thank you for working on this!

I'd like to see extra test coverage to ensure that an empty enum has the same ABI properties in Clang as it does in MSVC. e.g., the size of the enumeration is still the same as the size of an int, an object of that type is still passed as an int (no masking or other oddities), etc. So I think perhaps we should add a CodeGen test just to smoke test the behavior.

We should probably also mention the extension somewhere in clang/docs/LanguageExtensions.rst. We currently have a section for Enumerations with a fixed underlying type, so we could add one nearby for Enumerations with no enumerators or something.

AaronBallman · 2025-10-08T14:29:45Z

clang/test/Parser/ms-empty-enum.c

+
+typedef enum tag1 { } A; // expected-warning {{empty enumeration types are a Microsoft extension}}
+typedef enum tag2 { } B; // expected-warning {{empty enumeration types are a Microsoft extension}}
+typedef enum : unsigned { } C; // expected-warning {{enumeration types with a fixed underlying type are a Microsoft extension}}\


I think emitting two diagnostics is fine (one for the underlying type and one for the empty enum) because they are separate extensions. But I think it's a bit odd that we claim the underlying type is a Microsoft extension rather than a C23 extension -- that seems to be a preexisting issue which could be handled in a separate PR: https://godbolt.org/z/sMhGoE3qq

Signed-off-by: yicuixi <[email protected]>

yicuixi · 2025-10-20T16:22:32Z

Thank you for working on this!

I'd like to see extra test coverage to ensure that an empty enum has the same ABI properties in Clang as it does in MSVC. e.g., the size of the enumeration is still the same as the size of an int, an object of that type is still passed as an int (no masking or other oddities), etc. So I think perhaps we should add a CodeGen test just to smoke test the behavior.

We should probably also mention the extension somewhere in clang/docs/LanguageExtensions.rst. We currently have a section for Enumerations with a fixed underlying type, so we could add one nearby for Enumerations with no enumerators or something.

Thanks for the review and sorry for the very late update. I've added a new CodeGen test.

yicuixi · 2025-10-20T16:23:46Z

I think emitting two diagnostics is fine (one for the underlying type and one for the empty enum) because they are separate extensions. But I think it's a bit odd that we claim the underlying type is a Microsoft extension rather than a C23 extension -- that seems to be a preexisting issue which could be handled in a separate PR: https://godbolt.org/z/sMhGoE3qq

SGTM, I'd prefer to fix that in a separate patch.

Signed-off-by: yicuixi <[email protected]>

AaronBallman

Mostly LGTM aside from some nits with documentation

AaronBallman · 2025-10-23T14:53:39Z

clang/docs/LanguageExtensions.rst

+Enumerations with no enumerators
+-----------------------------------------


Suggested change

Enumerations with no enumerators

-----------------------------------------

Enumerations with no enumerators

--------------------------------

AaronBallman · 2025-10-23T14:54:04Z

clang/docs/LanguageExtensions.rst

+Enumerations with no enumerators
+-----------------------------------------
+
+Clang provides support for Microsoft extensions to support empty enums.


Suggested change

Clang provides support for Microsoft extensions to support empty enums.

Clang provides support for Microsoft extensions to support enumerations with no enumerators.

[clang] Accept empty enum in MSVC compatible C

77fc06c

Signed-off-by: yicuixi <[email protected]>

llvmbot added clang Clang issues not falling into any other category clang:frontend Language frontend issues, e.g. anything involving "Sema" labels Sep 21, 2025

yronglin requested review from AaronBallman, cor3ntin and erichkeane and removed request for cor3ntin and erichkeane September 21, 2025 14:25

Fix test

191eacf

Signed-off-by: yicuixi <[email protected]>

yronglin approved these changes Sep 21, 2025

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Sirraide reviewed Sep 21, 2025

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clang/lib/Parse/ParseDecl.cpp Outdated Show resolved Hide resolved

erichkeane requested changes Sep 22, 2025

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shafik reviewed Sep 23, 2025

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yicuixi and others added 2 commits September 25, 2025 01:31

Merge branch 'main' into ms-ext

e153bf2

address comments

c106aaa

yicuixi and others added 3 commits September 26, 2025 23:19

correct format

6bb40da

Merge branch 'main' into ms-ext

d9ab0fe

Merge branch 'main' into ms-ext

f573438

erichkeane reviewed Sep 29, 2025

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Update clang/docs/ReleaseNotes.rst

b1cbba5

address comments Co-authored-by: Erich Keane <[email protected]>

erichkeane reviewed Oct 7, 2025

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AaronBallman reviewed Oct 8, 2025

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Merge branch 'main' into ms-ext

4dfd0fd

[clang] Add CodeGen test to check the ABI compatibility

259e58d

Signed-off-by: yicuixi <[email protected]>

Fix test

394ae9c

Signed-off-by: yicuixi <[email protected]>

AaronBallman reviewed Oct 23, 2025

View reviewed changes

		Enumerations with no enumerators
		-----------------------------------------

	Clang provides support for Microsoft extensions to support empty enums.
	Clang provides support for Microsoft extensions to support enumerations with no enumerators.

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

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

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