[PATCH] libstdc++: Add missing std::tuple constructor [PR114147]

[Jonathan Wakely]

This fixes a regression on all active branches.

Tested aarch64-linux.  

-- >8 --

I caused a regression with commit r10-908 by adding a constraint to the
non-explicit allocator-extended default constructor, but seemingly
forgot to add an explicit overload with the corresponding constraint.

libstdc++-v3/ChangeLog:

        PR libstdc++/114147
        * include/std/tuple (tuple::tuple(allocator_arg_t, const Alloc&)):
        Add missing overload of allocator-extended default constructor.
        (tuple<T1,T2>::tuple(allocator_arg_t, const Alloc&)): Likewise.
        * testsuite/20_util/tuple/cons/114147.cc: New test.
---
 libstdc++-v3/include/std/tuple                    | 14 ++++++++++++++
 .../testsuite/20_util/tuple/cons/114147.cc        | 15 +++++++++++++++
 2 files changed, 29 insertions(+)
 create mode 100644 libstdc++-v3/testsuite/20_util/tuple/cons/114147.cc

diff --git a/libstdc++-v3/include/std/tuple b/libstdc++-v3/include/std/tuple
index 9c89c13ab84..3065058e184 100644
--- a/libstdc++-v3/include/std/tuple
+++ b/libstdc++-v3/include/std/tuple
@@ -1550,6 +1550,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
        tuple(allocator_arg_t __tag, const _Alloc& __a)
        : _Inherited(__tag, __a) { }
 
+      template<typename _Alloc,
+              _ExplicitDefaultCtor<is_object<_Alloc>::value> = false>
+       _GLIBCXX20_CONSTEXPR
+       explicit
+       tuple(allocator_arg_t __tag, const _Alloc& __a)
+       : _Inherited(__tag, __a) { }
+
       template<typename _Alloc, bool _NotEmpty = (sizeof...(_Elements) >= 1),
               _ImplicitCtor<_NotEmpty, const _Elements&...> = true>
        _GLIBCXX20_CONSTEXPR
@@ -2198,6 +2205,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
        tuple(allocator_arg_t __tag, const _Alloc& __a)
        : _Inherited(__tag, __a) { }
 
+      template<typename _Alloc,
+              _ExplicitDefaultCtor<is_object<_Alloc>::value, _T1, _T2> = false>
+       _GLIBCXX20_CONSTEXPR
+       explicit
+       tuple(allocator_arg_t __tag, const _Alloc& __a)
+       : _Inherited(__tag, __a) { }
+
       template<typename _Alloc, bool _Dummy = true,
               _ImplicitCtor<_Dummy, const _T1&, const _T2&> = true>
        _GLIBCXX20_CONSTEXPR
diff --git a/libstdc++-v3/testsuite/20_util/tuple/cons/114147.cc 
b/libstdc++-v3/testsuite/20_util/tuple/cons/114147.cc
new file mode 100644
index 00000000000..916e7204964
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/tuple/cons/114147.cc
@@ -0,0 +1,15 @@
+// { dg-do compile { target c++11 } }
+
+// PR libstdc++/114147
+// tuple allocator-extended ctor requires non-explicit default ctor
+
+#include <tuple>
+#include <memory>
+
+struct X { explicit X(); };
+
+std::allocator<int> a;
+std::tuple<X> t0(std::allocator_arg, a);
+std::tuple<int, X> t1(std::allocator_arg, a);
+std::tuple<X, int> t2(std::allocator_arg, a);
+std::tuple<int, X, int> t3(std::allocator_arg, a);
-- 
2.43.2