[patch] libstdc++/65978 LWG 2275 and 2301, constexpr forward_as_tuple and tie

[Jonathan Wakely]

Two easy DRs to implement. We even had the tests half written.

Tested x86_64-linux and powerpc64l2-linux, committed to trunk.

I'll probably backport this in a week or two.


commit c1a6989fbdad9870cdd2240d5fda58054d0ba5ae
Author: Jonathan Wakely <jwak...@redhat.com>
Date:   Sat May 2 11:29:17 2015 +0100

    	PR libstdc++/65978
    	* include/std/tuple (forward_as_tuple, tie): Add constexpr.
    	* testsuite/20_util/tuple/creation_functions/constexpr.cc: Uncomment
    	and fix tests for forward_as_tuple and tie.

diff --git a/libstdc++-v3/include/std/tuple b/libstdc++-v3/include/std/tuple
index e500a76..ad132bd 100644
--- a/libstdc++-v3/include/std/tuple
+++ b/libstdc++-v3/include/std/tuple
@@ -970,8 +970,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       return __result_type(std::forward<_Elements>(__args)...);
     }
 
+  // _GLIBCXX_RESOLVE_LIB_DEFECTS
+  // 2275. Why is forward_as_tuple not constexpr?
   template<typename... _Elements>
-    tuple<_Elements&&...>
+    constexpr tuple<_Elements&&...>
     forward_as_tuple(_Elements&&... __args) noexcept
     { return tuple<_Elements&&...>(std::forward<_Elements>(__args)...); }
 
@@ -1120,9 +1122,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       return __concater::_S_do(std::forward<_Tpls>(__tpls)...);
     }
 
+  // _GLIBCXX_RESOLVE_LIB_DEFECTS
+  // 2301. Why is tie not constexpr?
   /// tie
   template<typename... _Elements>
-    inline tuple<_Elements&...>
+    constexpr tuple<_Elements&...>
     tie(_Elements&... __args) noexcept
     { return tuple<_Elements&...>(__args...); }
 
diff --git a/libstdc++-v3/testsuite/20_util/tuple/creation_functions/constexpr.cc b/libstdc++-v3/testsuite/20_util/tuple/creation_functions/constexpr.cc
index 2c47fbb..3ededfc 100644
--- a/libstdc++-v3/testsuite/20_util/tuple/creation_functions/constexpr.cc
+++ b/libstdc++-v3/testsuite/20_util/tuple/creation_functions/constexpr.cc
@@ -45,49 +45,51 @@ test_make_tuple()
   }
 }
 
-#if 0
 // forward_as_tuple
 void
 test_forward_as_tuple()
 {
   {
-    typedef std::tuple<int, float> tuple_type;
+    static int i(22);
+    static float f(22.222);
+    typedef std::tuple<int&, float&&> tuple_type;
     constexpr tuple_type p1 __attribute__((unused))
-      = std::forward_as_tuple(22, 22.222);
+      = std::forward_as_tuple(i, std::move(f));
   }
 
   {
-    typedef std::tuple<int, float, int> tuple_type;
+    static int i(22);
+    static float f(22.222);
+    static int ii(77799);
+
+    typedef std::tuple<int&, float&, int&&> tuple_type;
     constexpr tuple_type p1 __attribute__((unused))
-      = std::forward_as_tuple(22, 22.222, 77799);
+      = std::forward_as_tuple(i, f, std::move(ii));
   }
 }
-#endif
 
-#if 0
 // tie
 void
 test_tie()
 {
   {
-    int i(22);
-    float f(22.222);
-    typedef std::tuple<int, float> tuple_type;
+    static int i(22);
+    static float f(22.222);
+    typedef std::tuple<int&, float&> tuple_type;
     constexpr tuple_type p1 __attribute__((unused))
       = std::tie(i, f);
   }
 
   {
-    int i(22);
-    float f(22.222);
-    int ii(77799);
+    static int i(22);
+    static float f(22.222);
+    static const int ii(77799);
 
-    typedef std::tuple<int, float, int> tuple_type;
+    typedef std::tuple<int&, float&, const int&> tuple_type;
     constexpr tuple_type p1 __attribute__((unused))
       = std::tie(i, f, ii);
   }
 }
-#endif
 
 // get
 void
@@ -124,6 +126,8 @@ int
 main()
 {
   test_make_tuple();
+  test_forward_as_tuple();
+  test_tie();
   test_get();
   test_tuple_cat();