[Bug libstdc++/110158] New: Cannot use union with std::string inside in constant expression

[Bug libstdc++/110158] New: Cannot use union with std::string inside in constant expression

[fchelnokov at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110158

            Bug ID: 110158
           Summary: Cannot use union with std::string inside in constant
                    expression
           Product: gcc
           Version: 13.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: fchelnokov at gmail dot com
  Target Milestone: ---

This program

constexpr bool f() {
    union U{
        std::string s;
        constexpr ~U(){ s.~basic_string(); }
    } u{};
    return true;
}

static_assert( f() );

is accepted in MSVC and Clang with libc++. But in GCC it produces the error:

error: non-constant condition for static assertion
   in 'constexpr' expansion of 'f()'
   in 'constexpr' expansion of '(& u)->f()::U::~U()'
   in 'constexpr' expansion of
'((f()::U*)this)->f()::U::s.std::__cxx11::basic_string<char>::~basic_string()'
/opt/compiler-explorer/gcc-13.1.0/include/c++/13.1.0/bits/basic_string.h:803:19:
  in 'constexpr' expansion of
'((std::__cxx11::basic_string<char>*)this)->std::__cxx11::basic_string<char>::_M_dispose()'
error: accessing 'std::__cxx11::basic_string<char>::<unnamed
union>::_M_allocated_capacity' member instead of initialized
'std::__cxx11::basic_string<char>::<unnamed union>::_M_local_buf' member in
constant expression

Online demo: https://gcc.godbolt.org/z/bbf4Yo3v9

[Bug c++/110158] Cannot use union with std::string inside in constant expression

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110158

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
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() );

```

[Bug c++/110158] Cannot use union with std::string inside in constant expression

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110158

Patrick Palka <ppalka at gcc dot gnu.org> 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 <ppalka at gcc dot gnu.org> ---
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

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110158

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |danakj at orodu dot net

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
*** Bug 110900 has been marked as a duplicate of this bug. ***

[Bug c++/110158] Cannot use union with std::string inside in constant expression

[danakj at orodu dot net]

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 <string>

template <class T, class I, class S, class F>
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() {}
```

<source>: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");
      |                    ~~~^~~~~~~~
<source>:18:32:   in 'constexpr' expansion of 'fold(T, I, S, F) [with T =
std::__cxx11::basic_string<char>; I = const char*; S = const char*; F =
<lambda(std::string, char)>](std::begin<const char, 3>(v), std::end<const char,
3>(v), (<lambda closure object><lambda(std::string, char)>(),
<lambda(std::string, char)>()))'
<source>:18:32:   in 'constexpr' expansion of
'std::__cxx11::basic_string<char>((* &
std::move<__cxx11::basic_string<char>&>(init)))'
<source>:18:23: error: accessing 'std::__cxx11::basic_string<char>::<unnamed
union>::_M_allocated_capacity' member instead of initialized
'std::__cxx11::basic_string<char>::<unnamed union>::_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

[danakj at orodu dot net]

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

[danakj at orodu dot net]

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 <string>

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:
```
<source>: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 '&S(("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 '&S(("hi")).s->_M_construct(&"hi"[0], &"hi"[2], {{}})'
        _M_construct(__s, __end, forward_iterator_tag());
        ^
<source>:11:21: note: in call to 'basic_string(&"hi"[0],
std::allocator<char>())'
static_assert(foo(S("hi")) == "hi");
                    ^
1 error generated.
```

[Bug c++/110158] Cannot use union with std::string inside in constant expression

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110158

--- Comment #7 from Jonathan Wakely <redi at gcc dot gnu.org> ---
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

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110158

--- Comment #8 from Patrick Palka <ppalka at gcc dot gnu.org> ---
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

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110158

--- Comment #9 from Jonathan Wakely <redi at gcc dot gnu.org> ---
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).