[PATCH] libstdc++: Implement LWG3528 make_from_tuple can perform (the equivalent of) a C-style cast

[Jonathan Wakely]

From: Yihan Wang <yronglin...@gmail.com>

Implement LWG3528 to make std::make_from_tuple SFINAE friendly.

libstdc++-v3/ChangeLog:

        * include/std/tuple (__can_make_from_tuple): New variable
        template.
        (__make_from_tuple_impl): Add static_assert.
        (make_from_tuple): Constrain using __can_make_from_tuple.
        * testsuite/20_util/tuple/dr3528.cc: New test.

Signed-off-by: Yihan Wang <yronglin...@gmail.com>
Co-authored-by: Jonathan Wakely <jwak...@redhat.com>
---

Here's a simpler version of Yihan's patch, with constraints on
std::make_from_tuple but not on __make_from_tuple_impl, and with an
original test rather than one copied from MSVC and libc++.

Lightly tested on x86_64-linux, full testing underway.

 libstdc++-v3/include/std/tuple                | 24 +++++++++-
 .../testsuite/20_util/tuple/dr3528.cc         | 44 +++++++++++++++++++
 2 files changed, 66 insertions(+), 2 deletions(-)
 create mode 100644 libstdc++-v3/testsuite/20_util/tuple/dr3528.cc

diff --git a/libstdc++-v3/include/std/tuple b/libstdc++-v3/include/std/tuple
index 2e69af13a98b..b39ce710984c 100644
--- a/libstdc++-v3/include/std/tuple
+++ b/libstdc++-v3/include/std/tuple
@@ -2939,19 +2939,39 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 #endif
 
 #ifdef __cpp_lib_make_from_tuple // C++ >= 17
+  template <typename _Tp, typename _Tuple, typename _Seq
+             = make_index_sequence<tuple_size_v<remove_reference_t<_Tuple>>>>
+    constexpr bool __can_make_from_tuple = false;
+
+  // _GLIBCXX_RESOLVE_LIB_DEFECTS
+  // 3528. make_from_tuple can perform (the equivalent of) a C-style cast
+  template <typename _Tp, typename _Tuple, size_t... _Idx>
+    constexpr bool __can_make_from_tuple<_Tp, _Tuple, index_sequence<_Idx...>>
+      = is_constructible_v<_Tp,
+                          decltype(std::get<_Idx>(std::declval<_Tuple>()))...>;
+
   template <typename _Tp, typename _Tuple, size_t... _Idx>
     constexpr _Tp
     __make_from_tuple_impl(_Tuple&& __t, index_sequence<_Idx...>)
-    { return _Tp(std::get<_Idx>(std::forward<_Tuple>(__t))...); }
+    {
+      static_assert(__can_make_from_tuple<_Tp, _Tuple, 
index_sequence<_Idx...>>);
+      return _Tp(std::get<_Idx>(std::forward<_Tuple>(__t))...);
+    }
 
 #if __cpp_lib_tuple_like // >= C++23
   template <typename _Tp, __tuple_like _Tuple>
 #else
   template <typename _Tp, typename _Tuple>
 #endif
-    constexpr _Tp
+    constexpr auto
     make_from_tuple(_Tuple&& __t)
     noexcept(__unpack_std_tuple<is_nothrow_constructible, _Tp, _Tuple>)
+#ifdef __cpp_concepts // >= C++20
+    -> _Tp
+    requires __can_make_from_tuple<_Tp, _Tuple>
+#else
+    -> __enable_if_t<__can_make_from_tuple<_Tp, _Tuple>, _Tp>
+#endif
     {
       constexpr size_t __n = tuple_size_v<remove_reference_t<_Tuple>>;
 #if __has_builtin(__reference_constructs_from_temporary)
diff --git a/libstdc++-v3/testsuite/20_util/tuple/dr3528.cc 
b/libstdc++-v3/testsuite/20_util/tuple/dr3528.cc
new file mode 100644
index 000000000000..a96c6b75e209
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/tuple/dr3528.cc
@@ -0,0 +1,44 @@
+// { dg-do compile { target c++17 } }
+
+// LWG 3528. make_from_tuple can perform (the equivalent of) a C-style cast
+
+#include <tuple>
+#include <array>
+#include <utility>
+
+template<typename T, typename Tuple>
+using make_t = decltype(std::make_from_tuple<T>(std::declval<Tuple>()));
+
+template<typename T, typename Tuple, typename = void>
+constexpr bool can_make = false;
+template<typename T, typename Tuple>
+constexpr bool can_make<T, Tuple, std::void_t<make_t<T, Tuple>>> = true;
+
+static_assert( can_make<int, std::tuple<int>> );
+static_assert( can_make<int, std::tuple<int>&> );
+static_assert( can_make<int, const std::tuple<int>&> );
+static_assert( can_make<int, std::array<short, 1>> );
+static_assert( can_make<int, const std::array<short, 1>&&> );
+static_assert( can_make<std::tuple<int, int>, std::pair<unsigned, long>> );
+static_assert( can_make<const int*, std::tuple<int*>> );
+static_assert( can_make<void*, std::tuple<int*>> );
+static_assert( can_make<int, std::tuple<>> );
+static_assert( ! can_make<int, std::tuple<int, int>> );
+static_assert( ! can_make<int, std::pair<short, char>> );
+static_assert( ! can_make<int, std::pair<short, char>&> );
+static_assert( ! can_make<int, std::tuple<const char*>> );
+static_assert( ! can_make<int*, std::tuple<const int*>> );
+static_assert( ! can_make<int*, std::tuple<void*>> );
+static_assert( ! can_make<int, std::array<int, 2>> );
+static_assert( ! can_make<void, std::tuple<>> );
+static_assert( ! can_make<void, std::array<int, 1>> );
+
+struct Two
+{
+  Two(const char*, int);
+};
+
+static_assert( can_make<Two, std::tuple<char*, unsigned>> );
+static_assert( ! can_make<Two, std::tuple<const char*, int, int>> );
+static_assert( can_make<Two, std::pair<const char*, long>> );
+static_assert( ! can_make<Two, std::pair<int*, long>> );
-- 
2.49.0