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.
git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@193206 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/clang/Basic/DiagnosticSemaKinds.td b/include/clang/Basic/DiagnosticSemaKinds.td
index f10cae4..7abb74c 100644
--- a/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/include/clang/Basic/DiagnosticSemaKinds.td
@@ -3581,6 +3581,9 @@
"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">;
diff --git a/lib/Sema/SemaDecl.cpp b/lib/Sema/SemaDecl.cpp
index 81ade1f..80f95e6 100644
--- a/lib/Sema/SemaDecl.cpp
+++ b/lib/Sema/SemaDecl.cpp
@@ -2607,9 +2607,22 @@
}
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)
diff --git a/test/SemaCXX/linkage-spec.cpp b/test/SemaCXX/linkage-spec.cpp
index bdc217d..1598d0e 100644
--- a/test/SemaCXX/linkage-spec.cpp
+++ b/test/SemaCXX/linkage-spec.cpp
@@ -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 @@
::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@-5 {{previous declaration is here}}
+// expected-warning@-3 {{retaining previous language linkage}}
+#endif
+ };
+ extern "C" int bar3(Foo *y) {
+ return y->x;
+ }
+ }
+}