On 9 March 2018 at 00:41, Jonathan Wakely wrote:
> In order to meet the total order requirements of [comparisons] p2 we
> need to cast unrelated pointers to uintptr_t before comparing them.
> Those casts aren't allowed in constant expressions, so only cast
> when __builtin_constant_p says the result of the comparison is not a
> compile-time constant (because the arguments are not constants, or the
> result of the comparison is unspecified). When the result is constant
> just compare the pointers directly without casting.
>
> This ensures that the function can be called in constant expressions
> with suitable arguments, but still yields a total order even for
> otherwise unspecified pointer comparisons.
>
>         PR libstdc++/78420
>         * include/bits/stl_function.h (__ptr_rel_ops): New struct providing
>         relational operators defining total order on pointers.
>         (greater<_Tp*>, less<_Tp*>, greater_equal<_Tp*>, less_equal<_Tp>*):
>         New partial specializations for pointers, using __ptr_rel_ops.
>         (greater<void>, less<void>, greater_equal<void>, less_equal<void*):
>         Dispatch to __ptr_rel_ops when both arguments are pointers.
>         * testsuite/20_util/function_objects/comparisons_pointer.cc: New.
>
> Tested powerpc64le-linux. I intend to commit this to trunk and
> backport to the active branches, unless anybody sees a problem with
> this solution, or has any suggestions for improvement.

I intentionally used "_Tp* const&" for the parameter types of the new
partial specializations like greater<_Tp*>, because that's what the
standard says for the primary templates. But it would be more
efficient to pass pointers by value, and 20.5.5.5 [member.functions]
p2 allows us to do that. Here's a new patch with that change.