[Bug c++/112642] ranges::fold_left tries to access inactive union member of string in constant expression
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112642 --- Comment #10 from Jonathan Wakely --- (In reply to Miro Palmu from comment #9) > Mine is 13.2.1 20230801 so way before Oct 21. (I did not know there were > different snapshots of the releases, I'm just a user trying to help :) ) 13.2.1 (and any x.y.1 version) is not a release, it's a snapshot made from a branch between releases. See https://gcc.gnu.org/develop.html#num_scheme or more details. Releases end with a .0 number. > > Anyway, the original GCC error is the same as PR 112642 > > You probably mean PR 110158 Oops! I meant PR 111258
[Bug c++/112642] ranges::fold_left tries to access inactive union member of string in constant expression
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112642 --- Comment #9 from Miro Palmu --- (In reply to Jonathan Wakely from comment #8) > > Also tried it locally with clang 16.0.6 with > > gcc-13.2.1 libstdc++ > > Which gcc-13.2.1 though? That's a snapshot that could date from any time in > the past four months. If I use gcc version 13.2.1 20231025 then clang > compiles it. Mine is 13.2.1 20230801 so way before Oct 21. (I did not know there were different snapshots of the releases, I'm just a user trying to help :) ) > Anyway, the original GCC error is the same as PR 112642 You probably mean PR 110158
[Bug c++/112642] ranges::fold_left tries to access inactive union member of string in constant expression
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112642 --- Comment #8 from Jonathan Wakely --- (In reply to Miro Palmu from comment #7) > (In reply to Jonathan Wakely from comment #6) > > The examples in comment 4 do compile using libstdc++ on clang, if you use > > libstdc++ headers from after sept 29 (for trunk) or oct 21 (for gcc-13). > > I was testing this on compiler explorer on clang 17.0.1 and it used > gcc-13.2.0 libstdc++. Which is expected to fail, because 13.2.0 was released before Oct 21. > Also tried it locally with clang 16.0.6 with > gcc-13.2.1 libstdc++ Which gcc-13.2.1 though? That's a snapshot that could date from any time in the past four months. If I use gcc version 13.2.1 20231025 then clang compiles it. Anyway, the original GCC error is the same as PR 112642 which was apparently reduced to PR 111284, which does seem relevant.
[Bug c++/112642] ranges::fold_left tries to access inactive union member of string in constant expression
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112642
--- Comment #7 from Miro Palmu ---
(In reply to Jonathan Wakely from comment #6)
> The examples in comment 4 do compile using libstdc++ on clang, if you use
> libstdc++ headers from after sept 29 (for trunk) or oct 21 (for gcc-13).
I was testing this on compiler explorer on clang 17.0.1 and it used gcc-13.2.0
libstdc++. Also tried it locally with clang 16.0.6 with gcc-13.2.1 libstdc++
Output:
$ cat prog.cpp
#include
#include
int main() {
[](std::string s = {}) consteval {
std::string ss{ std::move(s) };
}();
}
$ clang prog.cpp -std=c++2b -stdlib=libstdc++
prog.cpp:4:5: error: call to consteval function 'main()::(anonymous
class)::operator()' is not a constant expression
[](std::string s = {}) consteval {
^
/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/13.2.1/../../../../include/c++/13.2.1/bits/stl_construct.h:97:14:
note: construction of subobject of member '_M_local_buf' of union with no
active member is not allowed in a constant expression
{ return ::new((void*)__location) _Tp(std::forward<_Args>(__args)...); }
^
/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/13.2.1/../../../../include/c++/13.2.1/bits/char_traits.h:272:6:
note: in call to 'construct_at(_M_local_buf[0], s.._M_local_buf[0])'
std::construct_at(__s1 + __i, __s2[__i]);
^
/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/13.2.1/../../../../include/c++/13.2.1/bits/char_traits.h:443:11:
note: in call to 'copy(_M_local_buf[0], _M_local_buf[0], 1)'
return __gnu_cxx::char_traits::copy(__s1, __s2, __n);
^
/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/13.2.1/../../../../include/c++/13.2.1/bits/basic_string.h:672:6:
note: in call to 'copy(_M_local_buf[0], _M_local_buf[0], 1)'
traits_type::copy(_M_local_buf, __str._M_local_buf,
^
prog.cpp:5:21: note: in call to 'basic_string(s)'
std::string ss{ std::move(s) };
^
prog.cpp:4:5: note: in call to '&[](std::string s) {
std::string ss{std::move(s)};
}->operator()(}}, _M_local_buf[0]}, 0, {._M_local_buf = {0, 0, 0, 0, 0,
0, 0, 0, 0, 0, ...}}})'
[](std::string s = {}) consteval {
^
1 error generated.
[Bug c++/112642] ranges::fold_left tries to access inactive union member of string in constant expression
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112642 --- Comment #6 from Jonathan Wakely --- (In reply to Miro Palmu from comment #5) > If you use libstdc++ on clang these will not compile but with different > errors. The examples in comment 4 do compile using libstdc++ on clang, if you use libstdc++ headers from after sept 29 (for trunk) or oct 21 (for gcc-13).
[Bug c++/112642] ranges::fold_left tries to access inactive union member of string in constant expression
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112642
--- Comment #5 from Miro Palmu ---
I have been trying to figure out where exactly the bug is and these are my
findings.
> Or:
>
> #include
>
> consteval void bar() {
> auto _ = [](std::string s) { return s; }({});
> }
>
> int main() {
>bar();
> }
>
> Or:
>
> #include
> constexpr auto foo(std::string init) { return init; }
> constexpr auto bar() { return foo("").size(); }
> constexpr auto i = bar();
>
>Clang compiles both of these without problems (it can't compile anything using
>""s in a constant expression, maybe due to
>https://github.com/llvm/llvm-project/issues/68527)
>
> So I am pretty sure this is a g++ front end bug.
If you use libstdc++ on clang these will not compile but with different errors.
Then with following example I try to showcase the bug without std::string.
Try it out: https://godbolt.org/z/rvoeMEaxc
This is bare minimum of libstdc++ basic_string to reproduce this bug:
---
struct S {
union {
char a[1];
};
char* ptr;
constexpr S() : ptr{a} {
a[0] = {};
}
constexpr S(S&&) = delete;
constexpr S(const S&) = delete;
constexpr S operator=(S&&) = delete;
constexpr S operator=(const S&) = delete;
constexpr ~S() = default;
}
---
Then to reproduce the bug instance of this class has to be function parameter
and the function has to be constant evaluated.
This can happens in std::basic_string move constructor bits/basic_string.h:682
and following tester functions tries to emulate what happens in it.
---
// Should always be false
constexpr bool test(const S& s){
return s.ptr != s.a;
}
consteval void tester1(S param = {}) {
S notparam = {};
if (test(notparam)){
throw std::logic_error("compiletime notparam!");
}
if (test(param)) {
// gcc ends up here so fails to compile
// in std::basic_string move constructor
// compilation would fail due to accessing
// inactive union member
// clang and msvc never end up here
throw std::logic_error("compiletime param");
}
}
int main() { tester(); )
---
Notice that here only the parameter version fails.
In non-constant evaluated context (see godbolt link)
all of the test evaluate false as they should.
[Bug c++/112642] ranges::fold_left tries to access inactive union member of string in constant expression
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112642
Jonathan Wakely changed:
What|Removed |Added
Component|libstdc++ |c++
--- Comment #4 from Jonathan Wakely ---
Or:
#include
consteval void bar() {
auto _ = [](std::string s) { return s; }({});
}
int main() {
bar();
}
Or:
#include
constexpr auto foo(std::string init) { return init; }
constexpr auto bar() { return foo("").size(); }
constexpr auto i = bar();
Clang compiles both of these without problems (it can't compile anything using
""s in a constant expression, maybe due to
https://github.com/llvm/llvm-project/issues/68527)
So I am pretty sure this is a g++ front end bug.
