Its result indicates if the concept C is modelled by the collection
T... of types. A concept is a list of expressions that a type must
support. Concepts can refine other concepts to inherit all requirements
from those base concepts. Besides the concept check function itself, this contains
some helper functions of the form require* that transform
a non-matching condition to a substitution failure. E.g.,
requireConvertible<R>(t.f())
is not valid unless the result of t.f() is convertible to R.
Since models() is the user interface, it's contained in the
namespace Dune::. In contrast the various require*() functions
needed for concept definitions are in the namespace Dune::Concept::.
I strongly propose that concept definitions themselves also go into
namespaces Dune::(NonCoreModule::)Concept.
This merge request depends on the TypeList feature (!7 (merged)).
