[PATCH] libstdc++: Make CTAD ignore tuple(const Types&...) constructor [PR121771]

[Jonathan Wakely]

This is similar to the r16-3536-g0bb0d1d2880d56 change for std::pair, so
that CTAD ignores the tuple(const Types&...) constructor and only uses
the tuple(Types...) -> tuple<Types...> deduction guide. This ensures
that the deduced type comes from the decayed argument types.

libstdc++-v3/ChangeLog:

        PR libstdc++/121771
        * include/std/tuple (tuple::tuple(const Elements&...)): Use
        type_identity_t to prevent constructor being used for CTAD.
        (tuple::tuple(allocator_arg_t, const A&, const Elements&...)):
        Likewise.
        * testsuite/20_util/tuple/cons/121771.cc: New test.
---

Tested aarch64-linux.

 libstdc++-v3/include/std/tuple                | 12 ++---
 .../testsuite/20_util/tuple/cons/121771.cc    | 45 +++++++++++++++++++
 2 files changed, 51 insertions(+), 6 deletions(-)
 create mode 100644 libstdc++-v3/testsuite/20_util/tuple/cons/121771.cc

diff --git a/libstdc++-v3/include/std/tuple b/libstdc++-v3/include/std/tuple
index 7f8cb7cbb1f9..f7caa79cda04 100644
--- a/libstdc++-v3/include/std/tuple
+++ b/libstdc++-v3/include/std/tuple
@@ -969,7 +969,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       // Defined as a template to work around PR libstdc++/116440.
       template<typename = void>
        constexpr explicit(!__convertible<const _Elements&...>())
-       tuple(const _Elements&... __elements)
+       tuple(const type_identity_t<_Elements>&... __elements)
        noexcept(__nothrow_constructible<const _Elements&...>())
        requires (__constructible<const _Elements&...>())
        : _Inherited(__elements...)
@@ -1161,7 +1161,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       template<typename _Alloc>
        constexpr explicit(!__convertible<const _Elements&...>())
        tuple(allocator_arg_t __tag, const _Alloc& __a,
-             const _Elements&... __elements)
+             const type_identity_t<_Elements>&... __elements)
        requires (__constructible<const _Elements&...>())
        : _Inherited(__tag, __a, __elements...)
        { }
@@ -1470,14 +1470,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       template<bool _NotEmpty = (sizeof...(_Elements) >= 1),
               _ImplicitCtor<_NotEmpty, const _Elements&...> = true>
        constexpr
-       tuple(const _Elements&... __elements)
+       tuple(const __type_identity_t<_Elements>&... __elements)
        noexcept(__nothrow_constructible<const _Elements&...>())
        : _Inherited(__elements...) { }
 
       template<bool _NotEmpty = (sizeof...(_Elements) >= 1),
               _ExplicitCtor<_NotEmpty, const _Elements&...> = false>
        explicit constexpr
-       tuple(const _Elements&... __elements)
+       tuple(const __type_identity_t<_Elements>&... __elements)
        noexcept(__nothrow_constructible<const _Elements&...>())
        : _Inherited(__elements...) { }
 
@@ -1562,7 +1562,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
               _ImplicitCtor<_NotEmpty, const _Elements&...> = true>
        _GLIBCXX20_CONSTEXPR
        tuple(allocator_arg_t __tag, const _Alloc& __a,
-             const _Elements&... __elements)
+             const __type_identity_t<_Elements>&... __elements)
        : _Inherited(__tag, __a, __elements...) { }
 
       template<typename _Alloc, bool _NotEmpty = (sizeof...(_Elements) >= 1),
@@ -1570,7 +1570,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
        _GLIBCXX20_CONSTEXPR
        explicit
        tuple(allocator_arg_t __tag, const _Alloc& __a,
-             const _Elements&... __elements)
+             const __type_identity_t<_Elements>&... __elements)
        : _Inherited(__tag, __a, __elements...) { }
 
       template<typename _Alloc, typename... _UElements,
diff --git a/libstdc++-v3/testsuite/20_util/tuple/cons/121771.cc 
b/libstdc++-v3/testsuite/20_util/tuple/cons/121771.cc
new file mode 100644
index 000000000000..40bb37aced4f
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/tuple/cons/121771.cc
@@ -0,0 +1,45 @@
+// { dg-do compile { target c++17 } }
+
+// Bug 121771 - std::tuple CTAD fails for lvalue reference to function type
+
+#include <tuple>
+#include <memory>
+
+void func();
+
+std::tuple t(func);
+std::tuple<void (*)()>& r = t;
+
+struct Explicit {
+    Explicit();
+    explicit Explicit(const Explicit&);
+} ex;
+
+std::tuple t2(func, 1);
+std::tuple<void (*)(), int>& r2 = t2;
+
+std::tuple t2x(ex, func);
+std::tuple<Explicit, void (*)()>& r2x = t2x;
+
+std::tuple t3(1, func, 3);
+std::tuple<int, void (*)(), int>& r3 = t3;
+
+std::tuple t3x(ex, 2, func);
+std::tuple<Explicit, int, void (*)()>& r3x = t3x;
+
+std::allocator<int> alloc;
+
+std::tuple ta(std::allocator_arg, alloc, func);
+std::tuple<void (*)()>& ra = ta;
+
+std::tuple ta2(std::allocator_arg, alloc, func, 1);
+std::tuple<void (*)(), int>& ra2 = ta2;
+
+std::tuple ta2x(std::allocator_arg, alloc, ex, func);
+std::tuple<Explicit, void (*)()>& ra2x = ta2x;
+
+std::tuple ta3(std::allocator_arg, alloc, 1, func, 3);
+std::tuple<int, void (*)(), int>& ra3 = ta3;
+
+std::tuple ta3x(std::allocator_arg, alloc, ex, 2, func);
+std::tuple<Explicit, int, void (*)()>& ra3x = ta3x;
-- 
2.53.0