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.
---
Lightly tested so far on x86_64-linux, full tests running overnight.
This should be backported to all active branches.
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(-)
create mode 100644 libstdc++-v3/testsuite/20_util/pair/cons/110853.cc
diff --git a/libstdc++-v3/include/bits/stl_pair.h
b/libstdc++-v3/include/bits/stl_pair.h
index 661335b466a3..231d0bbd1f49 100644
--- a/libstdc++-v3/include/bits/stl_pair.h
+++ b/libstdc++-v3/include/bits/stl_pair.h
@@ -445,7 +445,7 @@ _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..1047590686d1
--- /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, value_twice);
+std::pair<int, void (*)()>& r = p;
--
2.51.0
