Lambda has no default constructor nor a copy or move assignment.
range\test\adaptors.cpp is checking that produced ranges are copiable,
but it is not doing so the for lambda, or lambda like objects.
default_constructible_unary_fn uses an optional for default
construction.
I extended it to use its emplace facility to copy assign non copy
assignable types.
(see boost::optional 'Type Requierment': 'If T is not MoveAssignable,
it is still possible to reset the value of optional<T> using function
emplace():')
Jean-Philippe DUFRAIGNE
1b4f8100ef