https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92193
Jonathan Wakely <redi at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |NEW
Last reconfirmed| |2019-11-29
Ever confirmed|0 |1
--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
This results in a far worse experience for users in C++20, where lots more
things become constexpr.
With the patch I suggested in Bug 92727 comment 4 the diagnostic in C++17 mode
would be:
In file included from /home/jwakely/gcc/10/include/c++/10.0.0/vector:67,
from up.cc:1:
/home/jwakely/gcc/10/include/c++/10.0.0/bits/stl_vector.h: In instantiation of
'void std::vector<_Tp, _Alloc>::push_back(const value_type&) [with _Tp = X;
_Alloc = std::allocator<X>; std::vector<_Tp, _Alloc>::value_type = X]':
up.cc:14:18: required from here
/home/jwakely/gcc/10/include/c++/10.0.0/bits/stl_vector.h:1188:18: error:
static assertion failed
1188 | static_assert(is_copy_constructible_v<value_type>);
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
But compiling the exact same code with -std=gnu++2a gives:
In file included from /home/jwakely/gcc/10/include/c++/10.0.0/vector:67,
from up.cc:1:
/home/jwakely/gcc/10/include/c++/10.0.0/bits/stl_vector.h: In instantiation of
'void std::vector<_Tp, _Alloc>::push_back(const value_type&) [with _Tp = X;
_Alloc = std::allocator<X>; std::vector<_Tp, _Alloc>::value_type = X]':
up.cc:14:18: required from here
/home/jwakely/gcc/10/include/c++/10.0.0/bits/stl_vector.h:1188:18: error:
static assertion failed
1188 | static_assert(is_copy_constructible_v<value_type>);
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from
/home/jwakely/gcc/10/include/c++/10.0.0/ext/alloc_traits.h:34,
from
/home/jwakely/gcc/10/include/c++/10.0.0/bits/stl_uninitialized.h:67,
from /home/jwakely/gcc/10/include/c++/10.0.0/vector:66,
from up.cc:1:
/home/jwakely/gcc/10/include/c++/10.0.0/bits/alloc_traits.h: In instantiation
of 'static constexpr void std::allocator_traits<std::allocator<_Up>
>::construct(std::allocator_traits<std::allocator<_Up> >::allocator_type&,
_Up*, _Args&& ...) [with _Up = X; _Args = {const X&}; _Tp = X;
std::allocator_traits<std::allocator<_Up> >::allocator_type =
std::allocator<X>]':
/home/jwakely/gcc/10/include/c++/10.0.0/bits/stl_vector.h:1193:30: required
from 'void std::vector<_Tp, _Alloc>::push_back(const value_type&) [with _Tp =
X; _Alloc = std::allocator<X>; std::vector<_Tp, _Alloc>::value_type = X]'
up.cc:14:18: required from here
/home/jwakely/gcc/10/include/c++/10.0.0/bits/alloc_traits.h:504:20: error: use
of deleted function 'X::X(const X&)'
504 | noexcept(noexcept(::new((void*)__p) _Up(std::declval<_Args>()...)))
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
up.cc:6:7: note: 'X::X(const X&)' is implicitly deleted because the default
definition would be ill-formed:
6 | class X {
| ^
up.cc:6:7: error: use of deleted function 'std::unique_ptr<_Tp,
_Dp>::unique_ptr(const std::unique_ptr<_Tp, _Dp>&) [with _Tp = Y; _Dp =
std::default_delete<Y>]'
In file included from /home/jwakely/gcc/10/include/c++/10.0.0/memory:82,
from up.cc:2:
/home/jwakely/gcc/10/include/c++/10.0.0/bits/unique_ptr.h:456:7: note: declared
here
456 | unique_ptr(const unique_ptr&) = delete;
| ^~~~~~~~~~
In file included from
/home/jwakely/gcc/10/include/c++/10.0.0/ext/alloc_traits.h:34,
from
/home/jwakely/gcc/10/include/c++/10.0.0/bits/stl_uninitialized.h:67,
from /home/jwakely/gcc/10/include/c++/10.0.0/vector:66,
from up.cc:1:
/home/jwakely/gcc/10/include/c++/10.0.0/bits/alloc_traits.h: In instantiation
of 'static constexpr void std::allocator_traits<std::allocator<_Up>
>::construct(std::allocator_traits<std::allocator<_Up> >::allocator_type&,
_Up*, _Args&& ...) [with _Up = X; _Args = {const X&}; _Tp = X;
std::allocator_traits<std::allocator<_Up> >::allocator_type =
std::allocator<X>]':
/home/jwakely/gcc/10/include/c++/10.0.0/bits/stl_vector.h:1193:30: required
from 'void std::vector<_Tp, _Alloc>::push_back(const value_type&) [with _Tp =
X; _Alloc = std::allocator<X>; std::vector<_Tp, _Alloc>::value_type = X]'
up.cc:14:18: required from here
/home/jwakely/gcc/10/include/c++/10.0.0/bits/alloc_traits.h:509:21: error: no
matching function for call to 'construct_at(X*&, const X&)'
509 | std::construct_at(__p, std::forward<_Args>(__args)...);
| ~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from
/home/jwakely/gcc/10/include/c++/10.0.0/bits/stl_tempbuf.h:60,
from
/home/jwakely/gcc/10/include/c++/10.0.0/bits/stl_algo.h:62,
from /home/jwakely/gcc/10/include/c++/10.0.0/vector:62,
from up.cc:1:
/home/jwakely/gcc/10/include/c++/10.0.0/bits/stl_construct.h:94:5: note:
candidate: 'template<class _Tp, class ... _Args> constexpr decltype
(::new(void*(0)) _Tp) std::construct_at(_Tp*, _Args&& ...)'
94 | construct_at(_Tp* __location, _Args&&... __args)
| ^~~~~~~~~~~~
/home/jwakely/gcc/10/include/c++/10.0.0/bits/stl_construct.h:94:5: note:
template argument deduction/substitution failed:
/home/jwakely/gcc/10/include/c++/10.0.0/bits/stl_construct.h: In substitution
of 'template<class _Tp, class ... _Args> constexpr decltype (::new(void*(0))
_Tp) std::construct_at(_Tp*, _Args&& ...) [with _Tp = X; _Args = {const X&}]':
/home/jwakely/gcc/10/include/c++/10.0.0/bits/alloc_traits.h:509:21: required
from 'static constexpr void std::allocator_traits<std::allocator<_Up>
>::construct(std::allocator_traits<std::allocator<_Up> >::allocator_type&,
_Up*, _Args&& ...) [with _Up = X; _Args = {const X&}; _Tp = X;
std::allocator_traits<std::allocator<_Up> >::allocator_type =
std::allocator<X>]'
/home/jwakely/gcc/10/include/c++/10.0.0/bits/stl_vector.h:1193:30: required
from 'void std::vector<_Tp, _Alloc>::push_back(const value_type&) [with _Tp =
X; _Alloc = std::allocator<X>; std::vector<_Tp, _Alloc>::value_type = X]'
up.cc:14:18: required from here
/home/jwakely/gcc/10/include/c++/10.0.0/bits/stl_construct.h:96:17: error: use
of deleted function 'X::X(const X&)'
96 | -> decltype(::new((void*)0) _Tp(std::declval<_Args>()...))
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
That's because in C++20 some of the code following the static assertion
involves a constexpr function, which calls another constexpr function, which
calls another ... and all of them get evaluated and barf out their errors. The
whole point of the static_assert is to stop that happening.
