[Bug c++/70248] constexpr initialization with unspecified equality expression accepted

[Bug c++/70248] constexpr initialization with unspecified equality expression accepted

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70248

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2020-10-15
           Severity|minor                       |normal
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW

--- Comment #7 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Another example with a relational operator:

constexpr const char* s = "";
constexpr const char* n = nullptr;
constexpr bool b = s > n;

The result of the comparison is unspecified because the pointers are not equal
and don't point to subobjects of the same object.

Clang, EDG and MSVC all reject it as expected.

[Bug c++/70248] constexpr initialization with unspecified equality expression accepted

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70248

--- Comment #8 from Jonathan Wakely <redi at gcc dot gnu.org> ---
In gcc/cp/typeck.c:cp_build_binary_op this warning should be an error during
constant evaluation for EQ_EXPR and NE_EXPR:

      if (complain & tf_warning)
        {
          tree stripped_orig_op0 = tree_strip_any_location_wrapper (orig_op0);
          tree stripped_orig_op1 = tree_strip_any_location_wrapper (orig_op1);
          if ((TREE_CODE (stripped_orig_op0) == STRING_CST
               && !integer_zerop (cp_fully_fold (op1)))
              || (TREE_CODE (stripped_orig_op1) == STRING_CST
                  && !integer_zerop (cp_fully_fold (op0))))
            warning_at (location, OPT_Waddress,
                        "comparison with string literal results in "
                        "unspecified behavior");
        }

And this one for rel ops and spaceships:

      if (TREE_CODE (orig_op0) == STRING_CST
          || TREE_CODE (orig_op1) == STRING_CST)
        {
          if (complain & tf_warning)
            warning_at (location, OPT_Waddress,
                        "comparison with string literal results "
                        "in unspecified behavior");
        }

That only handles the string literals cases though, we also need to reject
pointers to distinct objects at the same places.

[Bug c++/70248] constexpr initialization with unspecified equality expression accepted

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70248

--- Comment #9 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
*** Bug 85474 has been marked as a duplicate of this bug. ***

[Bug c++/70248] constexpr initialization with unspecified equality expression accepted

[pkeir at outlook dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70248

Paul Keir <pkeir at outlook dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |pkeir at outlook dot com

--- Comment #10 from Paul Keir <pkeir at outlook dot com> ---
Here is another example which GCC accepts, but Clang correctly rejects:

constexpr bool test()
{
  int arr[2]{};
  void *p1 = &arr[0];
  void *p2 = &arr[1];

  return p1 < p2;
}

constexpr bool b = test();

[Bug c++/70248] constexpr initialization with unspecified equality expression accepted

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70248

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|2020-10-15 00:00:00         |2024-1-3

--- Comment #11 from Jonathan Wakely <redi at gcc dot gnu.org> ---
And another:

struct S
{
    char data_[ 5 ];

    constexpr S( char const* p ) : data_()
    {
      if (p == data_)
        return;

      for (int i = 0; i < 5; ++i)
        data_[i] = p[i];
    }
};

constexpr S s( "test" );

[Bug c++/70248] constexpr initialization with unspecified equality expression accepted

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70248

--- Comment #12 from Jonathan Wakely <redi at gcc dot gnu.org> ---
If the comparison happens in a template, GCC rejects it (see PR 113200).

[Bug c++/70248] constexpr initialization with unspecified equality expression accepted

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70248

--- Comment #13 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Although this is accepted even though it's in a template, so it's more
complicated than just template vs non-template:

template<int N = 5>
consteval void f( char const* p )
{
  char buf[N]{};

  if (p == buf) // unspecified
    return;

  for (int i = 0; i < 5; ++i)
    buf[i] = p[i];
}

int main()
{
  f("test");
}