On Wed, Sep 3, 2025 at 3:41 AM Patrick Palka <ppa...@redhat.com> wrote:
> On Tue, 2 Sep 2025, Jonathan Wakely wrote: > > > 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. > LGTM > > Makes sense, IIUC the pair(T1, T2) guide will always be preferred over > the implicit pair(const T1&, const T2&) guide, so we might as well > disable the latter. The fewer implicit deduction guides to consider, > the better! > > (My understanding of the PR is that this is ultimately an implementation > bug rather than a specification bug, but this seems like a nice way to > sidestep the issue while making CTAD easier to reason about.) > I do not think this is implementation issue, the constrained vs unconstrained tie-breaker (https://eel.is/c++draft/over.match.best#general-2.5) is considered before implicit vs explicit deduction guide ( https://eel.is/c++draft/over.match.best#general-2.11), so deduction guides generated from constrained constructors should win over explicit ones, that are unconstrained. > > I think tuple has the same issue.. > > > > > 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 > > > > > >
