[Clang] handle both gnu and cpp11 attributes to ensure correct parsing inside extern block (#102864)

Fixes #101990

Author: a-tarasyuk

clang/docs/ReleaseNotes.rst

@@ -257,6 +257,7 @@ Bug Fixes to C++ Support
 - Properly reject defaulted relational operators with invalid types for explicit object parameters,
   e.g., ``bool operator==(this int, const Foo&)`` (#GH100329), and rvalue reference parameters.
 - Properly reject defaulted copy/move assignment operators that have a non-reference explicit object parameter.
+- Clang now properly handles the order of attributes in `extern` blocks. (#GH101990).
 - Fixed an assertion failure by preventing null explicit object arguments from being deduced. (#GH102025).
 
 Bug Fixes to AST Handling

clang/lib/Parse/ParseDeclCXX.cpp

@@ -425,7 +425,10 @@ Decl *Parser::ParseLinkage(ParsingDeclSpec &DS, DeclaratorContext Context) {
       [[fallthrough]];
     default:
       ParsedAttributes DeclAttrs(AttrFactory);
-      MaybeParseCXX11Attributes(DeclAttrs);
+      ParsedAttributes DeclSpecAttrs(AttrFactory);
+      while (MaybeParseCXX11Attributes(DeclAttrs) ||
+             MaybeParseGNUAttributes(DeclSpecAttrs))
+        ;
       ParseExternalDeclaration(DeclAttrs, DeclSpecAttrs);
       continue;
     }

clang/test/Parser/attr-order.cpp

@@ -31,3 +31,16 @@ template <int a>
 
 template <int a>
 [[noreturn]] __declspec(dllexport) __attribute__((cdecl)) void k(); // ok
+
+extern "C" {
+  __attribute__ ((__warn_unused_result__)) [[__maybe_unused__]] int l(int); // ok
+  [[__maybe_unused__]] __attribute__ ((__warn_unused_result__)) int m(int); // ok
+}
+
+extern "C" {
+  __attribute__ ((__warn_unused_result__)) [[__maybe_unused__]] int n (int); // ok
+  __attribute__ ((__warn_unused_result__)) [[__maybe_unused__]] static int o (int x) { return x; }; // ok
+}
+
+extern "C" __attribute__ ((__warn_unused_result__)) [[__maybe_unused__]] int p(int); // ok
+extern "C" [[__maybe_unused__]] __attribute__ ((__warn_unused_result__)) int q(int); // ok