https://gcc.gnu.org/bugzilla/show_bug.cgi?id=124444
Bug ID: 124444
Summary: debug _M_invalidate_if incompatible with C++20 since
r16-5845
Product: gcc
Version: 16.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: libstdc++
Assignee: unassigned at gcc dot gnu.org
Reporter: jakub at gcc dot gnu.org
Target Milestone: ---
#include <functional>
#include <iostream>
int main()
{
}
clang++ t.cc -g -std=c++20 -D_GLIBCXX_DEBUG
fails with:
In file included from t.cc:1:
In file included from
/usr/bin/../lib/gcc/x86_64-redhat-linux/16/../../../../include/c++/16/functional:81:
In file included from
/usr/bin/../lib/gcc/x86_64-redhat-linux/16/../../../../include/c++/16/vector:78:
In file included from
/usr/bin/../lib/gcc/x86_64-redhat-linux/16/../../../../include/c++/16/debug/vector:42:
In file included from
/usr/bin/../lib/gcc/x86_64-redhat-linux/16/../../../../include/c++/16/debug/safe_sequence.h:177:
/usr/bin/../lib/gcc/x86_64-redhat-linux/16/../../../../include/c++/16/debug/safe_sequence.tcc:46:27:
error: variable of non-literal
type '__gnu_cxx::__scoped_lock' cannot be defined in a constexpr function
before C++23
46 | __gnu_cxx::__scoped_lock sentry(this->_M_get_mutex());
| ^
/usr/bin/../lib/gcc/x86_64-redhat-linux/16/../../../../include/c++/16/ext/concurrence.h:233:9:
note: '__scoped_lock' is not literal
because it is not an aggregate and has no constexpr constructors other
than copy or move constructors
233 | class __scoped_lock
| ^
Wonder if for C++20 the body doesn't have to be moved to a non-constexpr
function template.
I think this is a bug in g++ that it accepts it.
struct S { S () : s (42) {} ~S () {} int s; };
#ifdef TMPL
template <int N>
#endif
constexpr bool
foo ()
{
if (__builtin_is_constant_evaluated ()) { return true; }
S s;
return true;
}
#ifdef TMPL
auto a = foo <42> ();
#else
auto a = foo ();
#endif
Both g++ and clang++ reject this with -std=c++20 if TMPL is not defined
g++ -S -std=c++20 -o /tmp/0.s /tmp/0.C
/tmp/0.C: In function ‘constexpr bool foo()’:
/tmp/0.C:9:5: error: variable ‘s’ of non-literal type ‘S’ in ‘constexpr’
function only available with ‘-std=c++23’ or ‘-std=gnu++23’
9 | S s;
| ^
/tmp/0.C:1:8: note: ‘S’ is not literal because:
1 | struct S { S () : s (42) {} ~S () {} int s; };
| ^
/tmp/0.C:1:8: note: ‘S’ does not have ‘constexpr’ destructor
clang++ -S -std=c++20 -o /tmp/0.s /tmp/0.C
/tmp/0.C:9:5: error: variable of non-literal type 'S' cannot be defined in a
constexpr function before C++23
9 | S s;
| ^
/tmp/0.C:1:8: note: 'S' is not literal because it is not an aggregate and has
no constexpr constructors other than copy or move constructors
1 | struct S { S () : s (42) {} ~S () {} int s; };
| ^
1 error generated.
but
g++ -S -DTMPL -std=c++20 -o /tmp/0.s /tmp/0.C
is quiet and
clang++ -S -DTMPL -std=c++20 -o /tmp/0.s /tmp/0.C
/tmp/0.C:9:5: error: variable of non-literal type 'S' cannot be defined in a
constexpr function before C++23
9 | S s;
| ^
/tmp/0.C:1:8: note: 'S' is not literal because it is not an aggregate and has
no constexpr constructors other than copy or move constructors
1 | struct S { S () : s (42) {} ~S () {} int s; };
| ^
/tmp/0.C:13:10: error: no matching function for call to 'foo'
13 | auto a = foo <42> ();
| ^~~~~~~~
/tmp/0.C:6:1: note: candidate template ignored: substitution failure [with N =
42]
6 | foo ()
| ^
2 errors generated.
