BOOST_CONCEPT_REQUIRES fails on g++

[john.femiani@…]

The mingw-g++ compiler seems to fail to prevent BOOST_CONCEPT_REQUIRES from working properly.

It is failing to prevent a function from being instantiated, and instead it is issuing an error.

This is being discussed on the newslist ​here.

This is a sample g++ error:

 g++ test.cpp 

test.cpp: In instantiation of 'DummyConcept<foo<int> >':
c:\mingw\bin\../lib/gcc/mingw32/4.3.0/../../../../include/boost/concept/detail/has_constraints.hpp:40:   instantiated from 'const bool boost::concept::not_satisfied<DummyConcept<foo<int> > >::value'
c:\mingw\bin\../lib/gcc/mingw32/4.3.0/../../../../include/boost/concept/detail/has_constraints.hpp:43:   instantiated from 'boost::concept::not_satisfied<DummyConcept<foo<int> > >'
c:\mingw\bin\../lib/gcc/mingw32/4.3.0/../../../../include/boost/mpl/if.hpp:67:   instantiated from 'boost::mpl::if_<boost::concept::not_satisfied<DummyConcept<foo<int> > >, boost::concept::constraint<DummyConcept<foo<int> > >, boost::concept::requirement<DummyConcept<foo<int> > > >'
c:\mingw\bin\../lib/gcc/mingw32/4.3.0/../../../../include/boost/concept/detail/general.hpp:19:   instantiated from 'boost::concept::requirement_<void (*)(DummyConcept<foo<int> >)>'
c:\mingw\bin\../lib/gcc/mingw32/4.3.0/../../../../include/boost/concept/requires.hpp:30:   instantiated from 'boost::_requires_<void (*)(DummyConcept<foo<int> >)>'
test.cpp:35:   instantiated from here
test.cpp:14: error: no type named 'something' in 'struct foo<int>'

[john.femiani@…]

Sourcecode that produces the error.

[Dave Abrahams]

The library is doing what it's supposed to do on GCC. In general, it isn't possible to check for concept conformance without causing an error, so BOOST_CONCEPT_REQUIRES is supposed to cause an error if conformance fails. It doesn't try to do SFINAE, because, in general, SFINAE is not possible.

I probably should have put this in the docs, but I didn't quite realize it until now: an overloaded function that uses BOOST_CONCEPT_REQUIRES should only require the least-refined concept in the overload set. Otherwise the concept check will cause compilation to fail if a more-refined model is passed. If you want to check for a more refined model, you need to use BOOST_CONCEPT_ASSERT((...)) in the function body.

[john]

If it is not doing SFINAE then why even have BOOST_CONCEPT_REQUIRES? Is it just to move the conditions to approximately where they would belong in C++0x?

[john]

Replying to john:

If it is not doing SFINAE then why even have BOOST_CONCEPT_REQUIRES? Is it just to move the conditions to approximately where they would belong in C++0x?

Don't bother to answer -- I know the answer.