[boost] review request: addition to type_traits library of is_less_comparable<T, U> and others

Subject: [boost] review request: addition to type_traits library of is_less_comparable<T, U> and others
From: Frédéric Bron (frederic.bron_at_[hidden])
Date: 2009-10-08 02:44:09

I would like to propose to your review the following addition to the
type_traits library (at
https://svn.boost.org/trac/boost/browser/sandbox/type_traits
and http://dl.free.fr/tK0BwIzYy).

The purpose of the addition is to add type traits to detect if types T and U
are comparable in the sens of <, <=, >, >=, == or != operators, i.e. if
t<u has a sens when t is of type T and u of type U (same for <=, >, >=, ==,
!=).

The following traits are added:

is_equal_to_comparable<T,U>
is_greater_comparable<T,U>
is_greater_equal_comparable<T,U>
is_less_comparable<T,U>
is_less_equal_comparable<T,U>
is_not_equal_to_comparable<T,U>

The names are based on the corresponding names of the standard
template library (<functional> header, section 20.3.3 of the
standard).

The code has the following properties:
* returns true if t<u is meaningful and returns a value convertible to bool
* returns false if t<u is meaningless.
* fails with compile time error if t<u is meaningful and returns void
(a possibility to avoid compile time error would be to return true
with an operator, trick but this has little sens as returning false
would be better)

The compilation has been tested with g++ 4.3.2 and icpc 10.0 on linux
64 bits, g++ 3.4.4 on cygwin, mingw32 3.4.4 on cygwin.

Regards,

Frédéric