[gcc r14-12001] libstdc++: Make CTAD ignore pair(const T1&, const T2&) constructor [PR110853]

[Jonathan Wakely]

https://gcc.gnu.org/g:98c53d4494eb38778c95414228625d2ed5a495e3

commit r14-12001-g98c53d4494eb38778c95414228625d2ed5a495e3
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Tue Sep 2 22:30:46 2025 +0100

    libstdc++: Make CTAD ignore pair(const T1&, const T2&) constructor [PR110853]
    
    For the pair(T1, T2) explicit deduction type to decay its arguments as
    intended, we need the pair(const T1&, const T2&) constructor to not be
    used for CTAD. Otherwise we try to instantiate pair<T1, T2> without
    decaying, which is ill-formed for function lvalues.
    
    Use std::type_identity_t<T1> to make the constructor unusable for an
    implicit deduction guide.
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/110853
            * include/bits/stl_pair.h [C++20] (pair(const T1&, const T2&)):
            Use std::type_identity_t<T1> for first parameter.
            * testsuite/20_util/pair/cons/110853.cc: New test.
    
    Reviewed-by: Patrick Palka <ppalka@redhat.com>
    Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>
    (cherry picked from commit 0bb0d1d2880d562298eeec8eee4ab4e8ba943260)

Diff:
---
 libstdc++-v3/include/bits/stl_pair.h               |  2 +-
 libstdc++-v3/testsuite/20_util/pair/cons/110853.cc | 10 ++++++++++
 2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/include/bits/stl_pair.h b/libstdc++-v3/include/bits/stl_pair.h
index 02f703237607..5f32e5bd2167 100644
--- a/libstdc++-v3/include/bits/stl_pair.h
+++ b/libstdc++-v3/include/bits/stl_pair.h
@@ -425,7 +425,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
       /// Constructor accepting lvalues of `first_type` and `second_type`
       constexpr explicit(!_S_convertible<const _T1&, const _T2&>())
-      pair(const _T1& __x, const _T2& __y)
+      pair(const type_identity_t<_T1>& __x, const _T2& __y)
       noexcept(_S_nothrow_constructible<const _T1&, const _T2&>())
       requires (_S_constructible<const _T1&, const _T2&>())
       : first(__x), second(__y)
diff --git a/libstdc++-v3/testsuite/20_util/pair/cons/110853.cc b/libstdc++-v3/testsuite/20_util/pair/cons/110853.cc
new file mode 100644
index 000000000000..57ebfb8d7daa
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/pair/cons/110853.cc
@@ -0,0 +1,10 @@
+// { dg-do compile { target c++17 } }
+// PR libstdc++/110853
+// Bad interaction between deduction guide with decay and constraints
+// (CTAD, std::pair and function lvalue)
+
+#include <utility>
+
+void func() {}
+std::pair p(1, func);
+std::pair<int, void (*)()>& r = p;