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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug libstdc++/103295] constexpr std::string does not work for clang
Date: Wed, 17 Nov 2021 09:53:58 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D103295

--- Comment #7 from Jonathan Wakely <redi at gcc dot gnu.org> ---
The code can be reduced to this, which GCC, EDG and MSVC all accept, but Cl=
ang
doesn't:

struct S
{
  union {
    char buf[8];
    char* ptr;
  };
  unsigned len;

  constexpr S(const char* s, unsigned n)
  {
    char* p;
    if (n > 7)
      p =3D ptr =3D new char[n];
    else
      p =3D buf;
    for (len =3D 0; len < n; ++len)
      p[len] =3D s[len];
    p[len] =3D '\0';
  }

  constexpr ~S()
  {
    if (len > 7)
      delete[] ptr;
  }
};

constexpr bool test()
{
  S s("test", 4);
  return true;
}

static_assert( test() );


Clang accepts it if written like this:

  constexpr S(const char* s, unsigned n)
  {
    if (n > max_size())
    {
      ptr =3D new char[n];
      for (len =3D 0; len < n; ++len)
        ptr[len] =3D s[len];
      ptr[len] =3D '\0';
    }
    else
    {
      for (len =3D 0; len < n; ++len)
        buf[len] =3D s[len];
      buf[len] =3D '\0';
    }
  }

It just doesn't allow assignment through the pointer.=