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

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]>
Co-authored-by: Erich Keane <[email protected]>
Co-authored-by: Aaron Ballman <[email protected]>

Author: 3 people

clang/docs/LanguageExtensions.rst

@@ -2409,6 +2409,16 @@ those modes.
 Use ``__has_feature(c_fixed_enum)`` to determine whether support for fixed
 underlying types is available in C23 and later.
 
+Enumerations with no enumerators
+--------------------------------
+
+Clang provides support for Microsoft extensions to support enumerations with no enumerators.
+
+.. code-block:: c++
+
+  typedef enum empty { } A;
+
+
 Interoperability with C++11 lambdas
 -----------------------------------
 

clang/docs/ReleaseNotes.rst

@@ -444,6 +444,7 @@ Bug Fixes in This Version
 - Fixed a failed assertion with empty filename arguments in ``__has_embed``. (#GH159898)
 - Fixed a failed assertion with empty filename in ``#embed`` directive. (#GH162951)
 - Fixed a crash triggered by unterminated ``__has_embed``. (#GH162953)
+- Accept empty enumerations in MSVC-compatible C mode. (#GH114402)
 
 Bug Fixes to Compiler Builtins
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

clang/include/clang/Basic/DiagnosticGroups.td

@@ -1436,6 +1436,7 @@ def MicrosoftDrectveSection : DiagGroup<"microsoft-drectve-section">;
 def MicrosoftInclude : DiagGroup<"microsoft-include">;
 def MicrosoftCppMacro : DiagGroup<"microsoft-cpp-macro">;
 def MicrosoftFixedEnum : DiagGroup<"microsoft-fixed-enum">;
+def MicrosoftEmptyEnum : DiagGroup<"microsoft-empty-enum">;
 def MicrosoftSealed : DiagGroup<"microsoft-sealed">;
 def MicrosoftAbstract : DiagGroup<"microsoft-abstract">;
 def MicrosoftUnqualifiedFriend : DiagGroup<"microsoft-unqualified-friend">;
@@ -1489,7 +1490,8 @@ def Microsoft : DiagGroup<"microsoft",
      MicrosoftConstInit, MicrosoftVoidPseudoDtor, MicrosoftAnonTag,
      MicrosoftCommentPaste, MicrosoftEndOfFile,
      MicrosoftInitFromPredefined, MicrosoftStringLiteralFromPredefined,
-     MicrosoftInconsistentDllImport, MicrosoftInlineOnNonFunction]>;
+     MicrosoftInconsistentDllImport, MicrosoftInlineOnNonFunction,
+     MicrosoftEmptyEnum]>;
 
 def ClangClPch : DiagGroup<"clang-cl-pch">;
 

clang/include/clang/Basic/DiagnosticParseKinds.td

@@ -116,6 +116,9 @@ def err_enumerator_unnamed_no_def : Error<
 def ext_ms_c_enum_fixed_underlying_type : Extension<
   "enumeration types with a fixed underlying type are a Microsoft extension">,
   InGroup<MicrosoftFixedEnum>;
+def ext_ms_c_empty_enum_type : Extension<
+  "empty enumeration types are a Microsoft extension">,
+  InGroup<MicrosoftEmptyEnum>;
 def ext_c23_enum_fixed_underlying_type : Extension<
   "enumeration types with a fixed underlying type are a C23 extension">,
   InGroup<C23>;

clang/lib/Parse/ParseDecl.cpp

@@ -5370,8 +5370,13 @@ 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)
-    Diag(Tok, diag::err_empty_enum);
+  if (Tok.is(tok::r_brace) && !getLangOpts().CPlusPlus) {
+    if (getLangOpts().MicrosoftExt)
+      Diag(T.getOpenLocation(), diag::ext_ms_c_empty_enum_type)
+          << SourceRange(T.getOpenLocation(), Tok.getLocation());
+    else
+      Diag(Tok, diag::err_empty_enum);
+  }
 
   SmallVector<Decl *, 32> EnumConstantDecls;
   SmallVector<SuppressAccessChecks, 32> EnumAvailabilityDiags;

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

@@ -0,0 +1,7 @@
+// RUN: %clang_cc1 -fms-extensions -triple x86_64-windows-msvc -Wno-implicit-function-declaration -emit-llvm %s -o - | FileCheck %s
+// RUN: %clang_cc1 -fms-extensions -triple i386-windows-msvc -Wno-implicit-function-declaration -emit-llvm %s -o - | FileCheck %s
+
+typedef enum tag1 {} A;
+
+// CHECK: void @empty_enum(i32 noundef %a)
+void empty_enum(A a) {}

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

@@ -0,0 +1,6 @@
+// RUN: %clang_cc1 %s -fsyntax-only -Wmicrosoft -verify -fms-extensions
+
+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}}\
+                               // expected-warning {{empty enumeration types are a Microsoft extension}}