libstdc++: Fix ODR violations caused by <tr1/functional>

The placeholders for std::tr1::bind are defined in an anonymous
namespace, which means they have internal linkage. This will cause ODR
violations when used in function templates (such as std::tr1::bind) from
multiple translation units. Although probably harmless (every definition
will generate identical code, even if technically ill-formed) we can
avoid the ODR violations by reusing the std::placeholder objects as the
std::tr1::placeholder objects.

To make this work, the std::_Placeholder type needs to be defined for
C++98 mode, so that <tr1/functional> can use it. The members of the
std::placeholder namespace must not be defined by <functional> in C++98
mode, because "placeholders", "_1", "_2" etc. are not reserved names in
C++98. Instead they can be declared in <tr1/functional>, because those
names *are* reserved in that header. With the std::placeholders objects
declared, a simple using-directive suffices to redeclare them in
namespace std::tr1::placeholders. This means any use of the TR1
placeholders actually refers to the C++11 placeholders, which are
defined with external linkage and exported from the library, so don't
cause ODR violations.

libstdc++-v3/ChangeLog:

	* include/std/functional (std::_Placeholder): Define for C++98
	as well as later standards.
	* include/tr1/functional (std::placeholders::_1 etc): Declare
	for C++98.
	(tr1::_Placeholder): Replace with using-declaration for
	std::_Placeholder.
	(tr1::placeholders::_1 etc.): Replace with using-directive for
	std::placeholders.

1 files changed, 10 insertions, 7 deletions

diff --git a/libstdc++-v3/include/std/functional b/libstdc++-v3/include/std/functional
index 9bad692f2ad..4e2d053f778 100644
--- a/libstdc++-v3/include/std/functional
+++ b/libstdc++-v3/include/std/functional
@@ -69,10 +69,19 @@
 # include <compare>
 #endif
 
+#endif // C++11
+
 namespace std _GLIBCXX_VISIBILITY(default)
 {
 _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
+  /** @brief The type of placeholder objects defined by libstdc++.
+   *  @ingroup binders
+   */
+  template<int _Num> struct _Placeholder { };
+
+#if __cplusplus >= 201103L
+
 #if __cplusplus >= 201703L
 # define __cpp_lib_invoke 201411L
 # if __cplusplus > 201703L
@@ -203,11 +212,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     = is_placeholder<_Tp>::value;
 #endif // C++17
 
-  /** @brief The type of placeholder objects defined by libstdc++.
-   *  @ingroup binders
-   */
-  template<int _Num> struct _Placeholder { };
-
   /** @namespace std::placeholders
    *  @brief ISO C++ 2011 namespace for std::bind placeholders.
    *  @ingroup binders
@@ -1271,10 +1275,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
 #endif // C++17
 #endif // C++14
+#endif // C++11
 
 _GLIBCXX_END_NAMESPACE_VERSION
 } // namespace std
 
-#endif // C++11
-
 #endif // _GLIBCXX_FUNCTIONAL