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.

llvm-svn: 193206

1 files changed, 16 insertions, 3 deletions

diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index 81ade1f3ce0..80f95e68690 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -2607,9 +2607,22 @@ bool Sema::MergeFunctionDecl(FunctionDecl *New, Decl *OldD, Scope *S,
     }
 
     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)