r193206 - Retain previous language linkage of friend function declarations

[Alp Toker]

Author: alp
Date: Tue Oct 22 17:53:01 2013
New Revision: 193206

URL: http://llvm.org/viewvc/llvm-project?rev=193206&view=rev
Log:
Retain previous language linkage of friend function declarations

With this extension, friend function declarations will retain the language
linkage specified for previous declarations instead of emitting an error
diagnostic.

The feature is known to be compatible with GCC and MSVC and permits a
language to be specified indirectly where it cannot otherwise be written
directly in class scope.

Work is ongoing to improve linkage spec diagnostics.

Fixes PR17337.

Reviewed by Richard Smith.

Modified:
    cfe/trunk/include/clang/Basic/DiagnosticSemaKinds.td
    cfe/trunk/lib/Sema/SemaDecl.cpp
    cfe/trunk/test/SemaCXX/linkage-spec.cpp

Modified: cfe/trunk/include/clang/Basic/DiagnosticSemaKinds.td
URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/include/clang/Basic/DiagnosticSemaKinds.td?rev=193206&r1=193205&r2=193206&view=diff
==============================================================================
--- cfe/trunk/include/clang/Basic/DiagnosticSemaKinds.td (original)
+++ cfe/trunk/include/clang/Basic/DiagnosticSemaKinds.td Tue Oct 22 17:53:01 2013
@@ -3581,6 +3581,9 @@ def err_static_non_static : Error<
   "static declaration of %0 follows non-static declaration">;
 def err_different_language_linkage : Error<
   "declaration of %0 has a different language linkage">;
+def ext_retained_language_linkage : Extension<
+  "friend function %0 retaining previous language linkage is an extension">,
+  InGroup<DiagGroup<"retained-language-linkage">>;
 def err_extern_c_global_conflict : Error<
   "declaration of %1 %select{with C language linkage|in global scope}0 "
   "conflicts with declaration %select{in global scope|with C language linkage}0">;

Modified: cfe/trunk/lib/Sema/SemaDecl.cpp
URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/Sema/SemaDecl.cpp?rev=193206&r1=193205&r2=193206&view=diff
==============================================================================
--- cfe/trunk/lib/Sema/SemaDecl.cpp (original)
+++ cfe/trunk/lib/Sema/SemaDecl.cpp Tue Oct 22 17:53:01 2013
@@ -2607,9 +2607,22 @@ bool Sema::MergeFunctionDecl(FunctionDec
     }
 
     if (haveIncompatibleLanguageLinkages(Old, New)) {
-      Diag(New->getLocation(), diag::err_different_language_linkage) << New;
-      Diag(Old->getLocation(), PrevDiag);
-      return true;
+      // As a special case, retain the language linkage from previous
+      // declarations of a friend function as an extension.
+      //
+      // This liberal interpretation of C++ [class.friend]p3 matches GCC/MSVC
+      // and is useful because there's otherwise no way to specify language
+      // linkage within class scope.
+      //
+      // Check cautiously as the friend object kind isn't yet complete.
+      if (New->getFriendObjectKind() != Decl::FOK_None) {
+        Diag(New->getLocation(), diag::ext_retained_language_linkage) << New;
+        Diag(Old->getLocation(), PrevDiag);
+      } else {
+        Diag(New->getLocation(), diag::err_different_language_linkage) << New;
+        Diag(Old->getLocation(), PrevDiag);
+        return true;
+      }
     }
 
     if (OldQTypeForComparison == NewQType)

Modified: cfe/trunk/test/SemaCXX/linkage-spec.cpp
URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/test/SemaCXX/linkage-spec.cpp?rev=193206&r1=193205&r2=193206&view=diff
==============================================================================
--- cfe/trunk/test/SemaCXX/linkage-spec.cpp (original)
+++ cfe/trunk/test/SemaCXX/linkage-spec.cpp Tue Oct 22 17:53:01 2013
@@ -1,4 +1,5 @@
 // RUN: %clang_cc1 -fsyntax-only -verify %s
+// RUN: %clang_cc1 -fsyntax-only -verify -Wretained-language-linkage -DW_RETAINED_LANGUAGE_LINKAGE  %s
 extern "C" {
   extern "C" void f(int);
 }
@@ -154,3 +155,21 @@ void bar_pr7927() {
   ::f_pr7927(E_7927);
   ::f_pr7927(0);
 }
+
+namespace PR17337 {
+  extern "C++" {
+    class Foo;
+    extern "C" int bar3(Foo *y);
+    class Foo {
+      int x;
+      friend int bar3(Foo *y);
+#ifdef W_RETAINED_LANGUAGE_LINKAGE
+// expected-note at -5 {{previous declaration is here}}
+// expected-warning at -3 {{retaining previous language linkage}}
+#endif
+    };
+    extern "C" int bar3(Foo *y) {
+      return y->x;
+    }
+  }
+}