https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112642
--- Comment #5 from Miro Palmu <miro.palmu at helsinki dot fi> ---
I have been trying to figure out where exactly the bug is and these are my
findings.
> Or:
>
> #include <string>
>
> consteval void bar() {
> auto _ = [](std::string s) { return s; }({});
> }
>
> int main() {
> bar();
> }
>
> Or:
>
> #include <string>
> 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.
