Make __wrap_iter work with gcc

[thakis]

Summary

The following snippet doesn't build when using gcc and libc++:

#include <string>
void f(const std::string& s) { s.begin(); }
#include <vector>
void AppendTo(const std::vector<char>& v) { v.begin(); }

The problem is that wrap_iter has a private constructor. It lists vector<> and basic_string<> as friends, but gcc seems to ignore this for vector<> for some reason. Declaring vector before the friend declaration in wrap_iter is enough to work around this problem, so do that. With this patch, I'm able to build chromium/android with libc++. Without it, two translation units fail to build.

As far as I can tell, this is due to a gcc bug, which I filed as https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64816.

Fixes PR22355.

[mclow.lists]

include/iterator

1116

There's also a friend decl for basic_string in __wrap_iter; why don't we need a fwd declaration for that as well?

(because it's a compiler bug?)

[thakis]

include/iterator

1116

I wondered about this too, but I wasn't able to tickle the symmetric problem for string. I think it's because iterator includes iosfwd, and iosfwd declares basic_string (as that file is where typedef basic_string<char> string lives, for some reason).

[thakis]

include/iterator

1116

In other words, there already is a forward declaration of basic_string somewhere.

[thakis]

Add _LIBCPP_TYPE_VIS_ONLY to forward decl.

[mclow.lists]

LGTM.

[thakis]

t227226, thanks!

include/iterator

 _LIBCPP_INLINE_VISIBILITY
 typename enable_if
 <
     is_trivially_copy_assignable<_Tp>::value,
     _Tp*
 >::type
 __unwrap_iter(__wrap_iter<_Tp*>);
 
+template <class _Tp, class _Alloc> class _LIBCPP_TYPE_VIS_ONLY vector;
+
 template <class _Iter>
 class __wrap_iter
 {
 public:
     typedef _Iter                                                      iterator_type;
     typedef typename iterator_traits<iterator_type>::iterator_category iterator_category;
     typedef typename iterator_traits<iterator_type>::value_type        value_type;
     typedef typename iterator_traits<iterator_type>::difference_type   difference_type;

test/std/iterators/iterators.general/gcc_workaround.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// Tests workaround for  https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64816.
+
+#include <string>
+
+void f(const std::string &s) { s.begin(); }
+
+#include <vector>
+
+void AppendTo(const std::vector<char> &v) { v.begin(); }
+
+int main() {}