Re: LessThanComparable requirement overspecified?

[Benjamin Kosnik <bkoz at redhat dot com>]

Matt I'm cc'ing you directly on this. Sorry if it's overkill.

> I noticed that the latest update of the `STL' part of libstdc++
> contains code to enforce the requirements on template parameters.
> This is quite useful --- i've already found several instances in my
> code where i was violating the spec.

Yeah it seemed like a good idea to me too. I'm not quite sure the work to 
get clean error messages is worth it, but enforcing invariants and 
requirements is a great idea.

Hey Gaby, any luck on getting human-readable error messages outta g++?

> However, there are some places where the new code seems to be overspecifying
> the requirements --- making demands not required by the C++ standard ---
> thus causing code that i believe to be legal to fail to compile.
> 
> Here is one example:
> 
> -----------------------------------------------------------------------
> #include <algorithm>
> 
> struct foo {};
> 
> bool operator== (const foo&, const foo&);
> bool operator< (const foo&, const foo&);
> 
> void bar (foo* a, foo* b, foo& x)
> {
>   foo* c = std::lower_bound (a, b, x);
> }
> 
> -----------------------------------------------------------------------
> 
> When i try to compile this, i get:
> 
> /usr/local/egcs/include/g++-v3/bits/concept_checks.h: In function `_Type
> _STL_ERROR::__less_than_comparable_requirement_violation (_Type, _Type) [with _Type = foo]':
> /usr/local/egcs/include/g++-v3/bits/concept_checks.h:560:   instantiated from `_LessThanComparable_concept_specification<_Type>::_LessThanComparable_requirement_violation (_Type) [with _Type = foo]'
> /usr/local/egcs/include/g++-v3/bits/stl_algo.h:1945:   instantiated from `std::lower_bound (_ForwardIter, _ForwardIter, const _Tp &) [with _ForwardIter = foo *, _Tp = foo]'
> x.cc:10:   instantiated from here
> /usr/local/egcs/include/g++-v3/bits/concept_checks.h:396: no match for `foo & >= foo &'
> /usr/local/egcs/include/g++-v3/bits/concept_checks.h:396: no match for `foo & <= foo &'
> /usr/local/egcs/include/g++-v3/bits/concept_checks.h:396: no match for `foo & > foo &'
> 
> 
> If i look at what __less_than_comparable_requirement is actually requiring:
> 
>     if (__a < __b || __a > __b || __a <= __b || __a >= __b) return __a;
> 
> i.e., it requires the full suite of four inequality comparisons to be
> present, not just `<'.
> 
> The requirement in the standard is that the type be LessThanComparable.
> According to 20.1.2, this requires only a `<' relation --- so i think
> that the code above is correct.

Hmm. Matt? 

> 
> This patch gets rid of the extra requirements, and allows the above
> example to compile.
> 
> 2000-06-29  scott snyder  <snyder@fnal.gov>
> 
> 	* bits/concept_checks.h
> 	(__less_then_comparable_requirement_violation): Only check for <.
> 
> Index: concept_checks.h
> ===================================================================
> RCS file: /cvs/gcc/egcs/libstdc++-v3/bits/concept_checks.h,v
> retrieving revision 1.1
> diff -u -p -c -p -r1.1 concept_checks.h
> *** concept_checks.h	2000/06/27 23:08:37	1.1
> --- concept_checks.h	2000/06/29 21:58:32
> *************** struct _STL_ERROR {
> *** 393,399 ****
>     template <class _Type>
>     static _Type
>     __less_than_comparable_requirement_violation(_Type __a, _Type __b) {
> !     if (__a < __b || __a > __b || __a <= __b || __a >= __b) return __a;
>       return __b;
>     }
>     template <class _Type>
> --- 393,399 ----
>     template <class _Type>
>     static _Type
>     __less_than_comparable_requirement_violation(_Type __a, _Type __b) {
> !     if (__a < __b) return __a;
>       return __b;
>     }
>     template <class _Type>
>