[PATCH] PR17337 - Retain previous linkage of friend function declarations

[Alp Toker]

The previous language linkage needs to be retained as described in
[class.friend]p3 otherwise valid friend redeclarations differing only in
lexical linkage are rejected.

Fixes PR17337.

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commit f694eb6a21cfbefe68ae45123f897c1de40a0ff5
Author: Alp Toker <alp at nuanti.com>
Date:   Tue Sep 24 10:28:40 2013 +0100

    Retain previous linkage of friend function declarations
    
    The previous language linkage needs to be retained as described in
    [class.friend]p3 otherwise valid friend redeclarations differing only in
    lexical linkage are rejected.
    
    Fixes PR17337.

diff --git a/lib/Sema/SemaDecl.cpp b/lib/Sema/SemaDecl.cpp
index 92400df..1b0a89f 100644
--- a/lib/Sema/SemaDecl.cpp
+++ b/lib/Sema/SemaDecl.cpp
@@ -2217,6 +2217,13 @@ static bool haveIncompatibleLanguageLinkages(const T *Old, const T *New) {
   if (DC->isRecord())
     return false;
 
+  // C++ [class.friend]p3:
+  //   A function first declared in a friend declaration has external linkage.
+  //   Otherwise, the function retains its previous linkage.
+  // The friend object kind is set later so we check the IDNS directly.
+  if (New->getIdentifierNamespace() & Decl::IDNS_OrdinaryFriend)
+    return false;
+
   LanguageLinkage OldLinkage = Old->getLanguageLinkage();
   if (OldLinkage == CXXLanguageLinkage && New->isInExternCContext())
     return true;
diff --git a/test/SemaCXX/linkage2.cpp b/test/SemaCXX/linkage2.cpp
index 075f5e7..dde239f 100644
--- a/test/SemaCXX/linkage2.cpp
+++ b/test/SemaCXX/linkage2.cpp
@@ -213,3 +213,17 @@ namespace PR16247 {
   void pr16247_bar(int) {}
   void pr16247_bar(double) {}
 }
+
+namespace PR17337 {
+  extern "C++" {
+    class Foo;
+    extern "C" int bar3(Foo *y);
+    class Foo {
+      int x;
+      friend int bar3(Foo *y);
+    };
+    extern "C" int bar3(Foo *y) {
+      return y->x;
+    }
+  }
+}