Allow 'inline' on some declarations in MS compatibility mode

[AaronBallman]

Microsoft allows the 'inline' specifier on a typedef of a function type
in C modes. This is used by a system header (ufxclient.h), so instead
of giving a hard error, we diagnose with a warning. C++ mode and non-
Microsoft compatibility modes are not impacted.

Fixes #124869

[llvmbot]

@llvm/pr-subscribers-clang

@llvm/pr-subscribers-platform-windows

Author: Aaron Ballman (AaronBallman)

Changes

Microsoft allows the 'inline' specifier on a typedef of a function type
in C modes. This is used by a system header (ufxclient.h), so instead
of giving a hard error, we diagnose with a warning. C++ mode and non-
Microsoft compatibility modes are not impacted.

Fixes #124869

---

Full diff: https://github.com/llvm/llvm-project/pull/125250.diff

6 Files Affected:

• (modified) clang/docs/ReleaseNotes.rst (+3)
• (modified) clang/include/clang/Basic/DiagnosticGroups.td (+3-1)
• (modified) clang/include/clang/Basic/DiagnosticSemaKinds.td (+2)
• (modified) clang/lib/Sema/SemaDecl.cpp (+4-1)
• (modified) clang/test/Sema/MicrosoftCompatibility.c (+14-2)
• (modified) clang/test/Sema/MicrosoftCompatibility.cpp (+7)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 8c290437fe16fe..a220e57d0b3222 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -83,6 +83,9 @@ Resolutions to C++ Defect Reports
 C Language Changes
 ------------------
 
+- Clang now allows an ``inline`` specifier on a typedef declaration of a
+  function type in Microsoft compatibility mode. #GH124869
+
 C2y Feature Support
 ^^^^^^^^^^^^^^^^^^^
 
diff --git a/clang/include/clang/Basic/DiagnosticGroups.td b/clang/include/clang/Basic/DiagnosticGroups.td
index 527e588d46a049..abb575002e1182 100644
--- a/clang/include/clang/Basic/DiagnosticGroups.td
+++ b/clang/include/clang/Basic/DiagnosticGroups.td
@@ -1304,6 +1304,8 @@ def MicrosoftStaticAssert : DiagGroup<"microsoft-static-assert">;
 def MicrosoftInitFromPredefined : DiagGroup<"microsoft-init-from-predefined">;
 def MicrosoftStringLiteralFromPredefined : DiagGroup<
     "microsoft-string-literal-from-predefined">;
+def MicrosoftInlineOnNonFunction : DiagGroup<
+    "microsoft-inline-on-non-function">;
 
 // Aliases.
 def : DiagGroup<"msvc-include", [MicrosoftInclude]>;
@@ -1322,7 +1324,7 @@ def Microsoft : DiagGroup<"microsoft",
      MicrosoftConstInit, MicrosoftVoidPseudoDtor, MicrosoftAnonTag,
      MicrosoftCommentPaste, MicrosoftEndOfFile, MicrosoftStaticAssert,
      MicrosoftInitFromPredefined, MicrosoftStringLiteralFromPredefined,
-     MicrosoftInconsistentDllImport]>;
+     MicrosoftInconsistentDllImport, MicrosoftInlineOnNonFunction]>;
 
 def ClangClPch : DiagGroup<"clang-cl-pch">;
 
diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index 2ac3879a4caabc..00a94eb7a30367 100644
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -482,6 +482,8 @@ def ext_use_out_of_scope_declaration : ExtWarn<
   InGroup<DiagGroup<"out-of-scope-function">>;
 def err_inline_non_function : Error<
   "'inline' can only appear on functions%select{| and non-local variables}0">;
+def warn_ms_inline_non_function : ExtWarn<err_inline_non_function.Summary>,
+  InGroup<MicrosoftInlineOnNonFunction>;
 def err_noreturn_non_function : Error<
   "'_Noreturn' can only appear on functions">;
 def warn_qual_return_type : Warning<
diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index f7b8b192a206c3..1755b37fc8f295 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -6681,7 +6681,10 @@ Sema::ActOnTypedefDeclarator(Scope* S, Declarator& D, DeclContext* DC,
   DiagnoseFunctionSpecifiers(D.getDeclSpec());
 
   if (D.getDeclSpec().isInlineSpecified())
-    Diag(D.getDeclSpec().getInlineSpecLoc(), diag::err_inline_non_function)
+    Diag(D.getDeclSpec().getInlineSpecLoc(),
+         (getLangOpts().MSVCCompat && !getLangOpts().CPlusPlus)
+             ? diag::warn_ms_inline_non_function
+             : diag::err_inline_non_function)
         << getLangOpts().CPlusPlus17;
   if (D.getDeclSpec().hasConstexprSpecifier())
     Diag(D.getDeclSpec().getConstexprSpecLoc(), diag::err_invalid_constexpr)
diff --git a/clang/test/Sema/MicrosoftCompatibility.c b/clang/test/Sema/MicrosoftCompatibility.c
index 9a1f050747f9d4..8d402d53e004d6 100644
--- a/clang/test/Sema/MicrosoftCompatibility.c
+++ b/clang/test/Sema/MicrosoftCompatibility.c
@@ -1,5 +1,5 @@
-// RUN: %clang_cc1 %s -fsyntax-only -Wno-unused-value -Wmicrosoft -verify -fms-compatibility -DMSVCCOMPAT -triple i686-pc-win32
-// RUN: %clang_cc1 %s -fsyntax-only -Wno-unused-value -Wmicrosoft -verify -fms-extensions -triple i686-pc-win32
+// RUN: %clang_cc1 %s -fsyntax-only -Wno-unused-value -Wmicrosoft -verify=expected,compat -fms-compatibility -DMSVCCOMPAT -triple i686-pc-win32
+// RUN: %clang_cc1 %s -fsyntax-only -Wno-unused-value -Wmicrosoft -verify=expected,ext -fms-extensions -triple i686-pc-win32
 
 #ifdef MSVCCOMPAT
 enum ENUM1; // expected-warning {{forward references to 'enum' types are a Microsoft extension}}
@@ -35,3 +35,15 @@ size_t x;
 #else
 size_t x; // expected-error {{unknown type name 'size_t'}}
 #endif
+
+/* Microsoft allows inline, __inline, and __forceinline to appear on a typedef
+   of a function type; this is used in their system headers such as ufxclient.h
+   See GitHub #124869 for more details.
+ */
+typedef int inline Foo1(int);       // compat-warning {{'inline' can only appear on functions}} \
+                                       ext-error {{'inline' can only appear on functions}}
+typedef int __inline Foo2(int);     // compat-warning {{'inline' can only appear on functions}} \
+                                       ext-error {{'inline' can only appear on functions}}
+typedef int __forceinline Foo(int); // compat-warning {{'inline' can only appear on functions}} \
+                                       ext-error {{'inline' can only appear on functions}} \
+                                       expected-warning {{'__forceinline' attribute only applies to functions and statements}}
diff --git a/clang/test/Sema/MicrosoftCompatibility.cpp b/clang/test/Sema/MicrosoftCompatibility.cpp
index 90a45dfaaf176d..391977e2765c5c 100644
--- a/clang/test/Sema/MicrosoftCompatibility.cpp
+++ b/clang/test/Sema/MicrosoftCompatibility.cpp
@@ -8,3 +8,10 @@ struct cls {
 };
 
 char * cls::* __uptr wrong2 = &cls::m; // expected-error {{'__uptr' attribute cannot be used with pointers to members}}
+
+// Microsoft allows inline, __inline, and __forceinline to appear on a typedef
+// of a function type, but only in C. See GitHub #124869 for more details.
+typedef int inline Foo1(int);       // expected-error {{'inline' can only appear on functions}}
+typedef int __inline Foo2(int);     // expected-error {{'inline' can only appear on functions}}
+typedef int __forceinline Foo(int); // expected-error {{'inline' can only appear on functions}} \
+                                       expected-warning {{'__forceinline' attribute only applies to functions and statements}}

[AaronBallman]

/cherry-pick ef91cae

[llvmbot]

Failed to cherry-pick: ef91cae

https://github.com/llvm/llvm-project/actions/runs/13079539011

Please manually backport the fix and push it to your github fork. Once this is done, please create a pull request