From 14318c6aa40d7c68c4341f43fbc3d0980f6d21a8 Mon Sep 17 00:00:00 2001
From: Serge Pavlov <sepavloff@gmail.com>
Date: Thu, 8 Jun 2017 06:07:07 +0000
Subject: [PATCH] Improve diagnostics if friend function redefines file-level
 function.

Clang makes check for function redefinition after it merged the new
declaration with the existing one. As a result, it produces poor
diagnostics in the case of a friend function defined inline, as in
the code:
```
    void func() {}
    class C { friend void func() {} };
```
Error message in this case states that `inline declaration of 'func'
follows non-inline definition`, which is misleading, as `func` does
not have explicit `inline` specifier.

With this changes compiler reports function redefinition if the new
function is a friend defined inline and it does not have explicit
`inline` specifier.

Differential Revision: https://reviews.llvm.org/D26065


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@304964 91177308-0d34-0410-b5e6-96231b3b80d8
---
 lib/Sema/SemaDeclCXX.cpp                      |  7 ++++-
 test/CXX/dcl.dcl/dcl.spec/dcl.fct.spec/p4.cpp | 28 +++++++++++++++++++
 2 files changed, 34 insertions(+), 1 deletion(-)

diff --git a/lib/Sema/SemaDeclCXX.cpp b/lib/Sema/SemaDeclCXX.cpp
index 3e2306361c..313552e23b 100644
--- a/lib/Sema/SemaDeclCXX.cpp
+++ b/lib/Sema/SemaDeclCXX.cpp
@@ -638,7 +638,12 @@ bool Sema::MergeCXXFunctionDecl(FunctionDecl *New, FunctionDecl *Old,
     Diag(Old->getLocation(), diag::note_previous_declaration);
     Invalid = true;
   } else if (!Old->getMostRecentDecl()->isInlined() && New->isInlined() &&
-             Old->isDefined(Def)) {
+             Old->isDefined(Def) &&
+             // If a friend function is inlined but does not have 'inline'
+             // specifier, it is a definition. Do not report attribute conflict
+             // in this case, redefinition will be diagnosed later.
+             (New->isInlineSpecified() ||
+              New->getFriendObjectKind() == Decl::FOK_None)) {
     // C++11 [dcl.fcn.spec]p4:
     //   If the definition of a function appears in a translation unit before its
     //   first declaration as inline, the program is ill-formed.
diff --git a/test/CXX/dcl.dcl/dcl.spec/dcl.fct.spec/p4.cpp b/test/CXX/dcl.dcl/dcl.spec/dcl.fct.spec/p4.cpp
index 12237a39ab..6e150ce5a5 100644
--- a/test/CXX/dcl.dcl/dcl.spec/dcl.fct.spec/p4.cpp
+++ b/test/CXX/dcl.dcl/dcl.spec/dcl.fct.spec/p4.cpp
@@ -4,3 +4,31 @@ void f0() { // expected-note {{previous definition is here}}
 }
 
 inline void f0(); // expected-error {{inline declaration of 'f0' follows non-inline definition}}
+
+void func_01() {} // expected-note{{previous definition is here}}
+struct C01 {
+  friend void func_01() {} // expected-error{{redefinition of 'func_01'}}
+};
+
+void func_02() {} // expected-note{{previous definition is here}}
+struct C02 {
+  friend inline void func_02(); // expected-error{{inline declaration of 'func_02' follows non-inline definition}}
+};
+
+void func_03() {} // expected-note{{previous definition is here}}
+struct C03 {
+  friend inline void func_03() {} // expected-error{{inline declaration of 'func_03' follows non-inline definition}}
+};
+
+void func_04() {} // expected-note{{previous definition is here}}
+inline void func_04() {} // expected-error{{inline declaration of 'func_04' follows non-inline definition}}
+
+void func_06() {} // expected-note{{previous definition is here}}
+template<typename T> struct C06 {
+  friend inline void func_06() {} // expected-error{{inline declaration of 'func_06' follows non-inline definition}}
+};
+
+void func_07() {} // expected-note{{previous definition is here}}
+template<typename T> struct C07 {
+  friend inline void func_07(); // expected-error{{inline declaration of 'func_07' follows non-inline definition}}
+};
-- 
2.40.0