[Bug c++/110158] Cannot use union with std::string inside in constant expression
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110158 --- Comment #9 from Jonathan Wakely --- Odd, I thought I'd checked it when testing r14-4334-g28adad7a32ed92. Seems like the same issue as PR 112642 though (which has a minimized version without std::string).
[Bug c++/110158] Cannot use union with std::string inside in constant expression
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110158 --- Comment #8 from Patrick Palka --- It seems the comment #4 testcase is still rejected on trunk after/despite r14-4334-g28adad7a32ed92?
[Bug c++/110158] Cannot use union with std::string inside in constant expression
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110158 --- Comment #7 from Jonathan Wakely --- That bug in comment 6 has already been fixed in gcc-12.3, by the last patch for PR 103295. The one in comment 4 is different, and will be fixed by Nathaniel's latest patch: https://gcc.gnu.org/pipermail/gcc-patches/2023-September/631642.html
[Bug c++/110158] Cannot use union with std::string inside in constant expression
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110158
--- Comment #6 from danakj at orodu dot net ---
In case it is of help, here's an even smaller repro that clang reports the
error in libstdc++. For whatever reason gcc does not notice the libstdc++ bug
here.
https://gcc.godbolt.org/z/Phndafeoe
```
#include
struct S {
constexpr ~S() {}
std::string s;
};
constexpr std::string foo(const S& s) { return s.s; }
static_assert(foo(S("hi")) == "hi"); // Fails.
static_assert(foo(S("a longer string works")) == "a longer string works");
int main() {}
```
The error:
```
:11:15: error: static assertion expression is not an integral constant
expression
static_assert(foo(S("hi")) == "hi");
^~~~
/opt/compiler-explorer/gcc-12.2.0/lib/gcc/x86_64-linux-gnu/12.2.0/../../../../include/c++/12.2.0/bits/basic_string.h:356:10:
note: assignment to member '_M_local_buf' of union with no active member is not
allowed in a constant expression
__c = _CharT();
^
/opt/compiler-explorer/gcc-12.2.0/lib/gcc/x86_64-linux-gnu/12.2.0/../../../../include/c++/12.2.0/bits/basic_string.tcc:229:4:
note: in call to '(("hi")).s->_M_use_local_data()'
_M_use_local_data();
^
/opt/compiler-explorer/gcc-12.2.0/lib/gcc/x86_64-linux-gnu/12.2.0/../../../../include/c++/12.2.0/bits/basic_string.h:642:2:
note: in call to '(("hi")).s->_M_construct(&"hi"[0], &"hi"[2], {{}})'
_M_construct(__s, __end, forward_iterator_tag());
^
:11:21: note: in call to 'basic_string(&"hi"[0],
std::allocator())'
static_assert(foo(S("hi")) == "hi");
^
1 error generated.
```
[Bug c++/110158] Cannot use union with std::string inside in constant expression
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110158 --- Comment #5 from danakj at orodu dot net --- As a means to work around #4 in static asserts, making your string long enough that it avoids the SSO will allow the compiler to accept it.
[Bug c++/110158] Cannot use union with std::string inside in constant expression
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110158
--- Comment #4 from danakj at orodu dot net ---
Here's a repro without the std::string inside a union. It is the SSO union
inside the string that causes the error.
https://gcc.godbolt.org/z/T8oM8vYnq
```
#include
template
constexpr T fold(T init, I i, S s, F f) {
while (true) {
if (i == s)
return init;
else
init = f(std::move(init), *i++);
}
}
constexpr char v[] = {'a', 'b', 'c'};
static_assert(fold(std::string(), std::begin(v), std::end(v),
[](std::string acc, char v) {
acc.push_back(v);
return acc;
}) == "abc");
int main() {}
```
:18:23: error: non-constant condition for static assertion
14 | static_assert(fold(std::string(), std::begin(v), std::end(v),
| ~~~
15 |[](std::string acc, char v) {
|~
16 |acc.push_back(v);
|~
17 |return acc;
|~~~
18 |}) == "abc");
|~~~^~~~
:18:32: in 'constexpr' expansion of 'fold(T, I, S, F) [with T =
std::__cxx11::basic_string; I = const char*; S = const char*; F =
](std::begin(v), std::end(v), ((),
()))'
:18:32: in 'constexpr' expansion of
'std::__cxx11::basic_string((* &
std::move<__cxx11::basic_string&>(init)))'
:18:23: error: accessing 'std::__cxx11::basic_string_M_allocated_capacity' member instead of initialized
'std::__cxx11::basic_string_M_local_buf' member in
constant expression
ASM generation compiler returned: 1
[Bug c++/110158] Cannot use union with std::string inside in constant expression
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110158 Andrew Pinski changed: What|Removed |Added CC||danakj at orodu dot net --- Comment #3 from Andrew Pinski --- *** Bug 110900 has been marked as a duplicate of this bug. ***
[Bug c++/110158] Cannot use union with std::string inside in constant expression
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110158
Patrick Palka changed:
What|Removed |Added
CC||ppalka at gcc dot gnu.org
See Also||https://gcc.gnu.org/bugzill
||a/show_bug.cgi?id=110245
--- Comment #2 from Patrick Palka ---
It works if we activate the union's member:
constexpr bool f() {
union U{
std::string s = {};
constexpr ~U(){ s.~basic_string(); }
} u{};
return true;
}
static_assert( f() );
So perhaps this is a dup of PR110245?
[Bug c++/110158] Cannot use union with std::string inside in constant expression
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110158
--- Comment #1 from Andrew Pinski ---
Here is a slightly reduced testcase (for a slightly different issue still
dealing with unions):
```
struct str1
{
// bool a;
char *var;
union {
char t[15];
int allocated;
};
constexpr str1() : var(new char[2]) { t[0] = 0; }
constexpr ~str1() {if (var != t) delete[] var; }
};
typedef str1 str;
constexpr bool f1() {
str t{};
return true;
}
static_assert( f1() );
constexpr bool f() {
union U{
str s;
constexpr ~U(){ s.~str(); }
} u{};
return true;
}
static_assert( f() );
```
