https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119866
Bug ID: 119866
Summary: constant evaluation failure with trivial function
Product: gcc
Version: 15.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: barry.revzin at gmail dot com
Target Milestone: ---
Not sure how to exactly title this bug.
This is a reproduction while trying to understand a compile failure with a
recent upgrade to {fmt}:
using size_t = decltype(sizeof(int));
constexpr char const* id(char const* s) { return s; }
template <size_t N>
struct fixed_string {
char data[N] = {};
constexpr fixed_string(char const (&m)[N]) {
for (size_t i = 0; i != N; ++i) {
data[i] = m[i];
}
}
};
int main() {
static constexpr char msg[] = "hello\n";
static_assert(__builtin_strlen(msg) == 6); // ok
static_assert(__builtin_strlen(id(msg)) == 6); // ok
static constexpr auto str = fixed_string("hello\n");
static_assert(__builtin_strlen(str.data) == 6); // ok
static_assert(__builtin_strlen(auto(str.data)) == 6); // ok
static_assert(__builtin_strlen(id(str.data)) == 6); // error
}
The last assertion fails (on trunk c++26, but also lots of versions going
back):
<source>:24:50: error: non-constant condition for static assertion
24 | static_assert(__builtin_strlen(id(str.data)) == 6); // error
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~
<source>:24:35: error: '__builtin_strlen(((const char*)(&
str.fixed_string<7>::data)))' is not a constant expression
24 | static_assert(__builtin_strlen(id(str.data)) == 6); // error
| ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~
id(x) is just x, it's not doing anything to the parameter. It's like the
constant evaluator loses track of the pointer.
