[Bug c++/87699] Implement CWG 1512

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87699

--- Comment #7 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Marek Polacek <mpola...@gcc.gnu.org>:

https://gcc.gnu.org/g:ae8ed736addb2b005d54c0b3191ac599a04ec170

commit r11-467-gae8ed736addb2b005d54c0b3191ac599a04ec170
Author: Marek Polacek <pola...@redhat.com>
Date:   Wed May 13 15:52:42 2020 -0400

    c++: Implement DR 1512, Pointer comparison vs qual convs [PR87699]

    This patch resolves DR 1512 (and, by turn, DR 583).  This entails:

    1) Relational pointer comparisons against null pointer constants have
       been made ill-formed:

       void f(char *p) {
          if (p > 0)
            // ...
       }

       was always invalid in C but was -- accidentally -- allowed in C++.

    2) This was ill-formed:

       bool foo(int** x, const int** y) {
         return x < y;
       }

       because 'int**' couldn't be converted to 'const int**'.  This was
       fixed by re-defining a generic composite pointer type.  The composite
       type of these two pointers will be 'const int *const *', to which
       both pointers can be converted.

    3) The overload descriptions for built-in operators were adjusted,
       because objects of type std::nullptr_t cannot be used with relational
       operators any more.

    I fixed 1) by adjusting cp_build_binary_op; we already had a warning
    for it so made it a hard error now.

    Then 2) required tweaking composite_pointer_type_r.  [expr.type] defines
    the composite pointer type by using the "cv-combined type."  We didn't
    implement the [conv.qual]/3.3 part; previously the composite type of
    'int**' and 'const int**' was 'const int**', so this didn't compile:

        void f(const int **p, int **q) {
          true ? p : q;
        }

    I wrote a more extensive test for this which uses decltype and some
    template magic to check the composite type, see composite-ptr-type.C.
    We still don't handle everything that [expr.type] requires us to,
    but it's pretty close.

    And finally 3) was handled in add_builtin_candidate.  Turned out we
    weren't creating built-in operator candidates when the type was
    std::nullptr_t at all.  We should, for == and !=.  Tested in builtin4.C.
    In passing, I'm fixing some of the comments too.

            DR 1512
            PR c++/87699
            * call.c (add_builtin_candidate) <case EQ_EXPR>: Create candidate
            operator functions when type is std::nullptr_t for ==/!=.
            * typeck.c (composite_pointer_type_r): Add bool a * parameter.  Use
it
            to maybe add "const" to the pointer type.
            (composite_pointer_type): Update the call to
composite_pointer_type_r.
            (cp_build_binary_op): Turn two warning_at into error_at.  Print the
            types.

            * g++.dg/cpp0x/constexpr-array-ptr10.C: Change dg-warning to
dg-error
            and adjust the expected messages in dg-error.
            * g++.dg/expr/composite-ptr-type.C: New test.
            * g++.dg/expr/ptr-comp1.C: New test.
            * g++.dg/expr/ptr-comp2.C: New test.
            * g++.dg/expr/ptr-comp3.C: New test.
            * g++.dg/overload/builtin4.C: New test.
            * g++.dg/warn/Wextra-3.C: Change dg-warning to dg-error.