Allow 'inline' on some declarations in MS compatibility mode (#125250)

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 https://github.com/llvm/llvm-project/issues/124869

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
 ^^^^^^^^^^^^^^^^^^^
 

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">;
 

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<

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)

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}}

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}}