...

I guess we have to wait for decltype to be able to use lambda expressions as predicates here?

Why? There are already examples with lambda predicates in the docs.

Yes, constrained_value does support lambdas at runtime using runtime boost::function. I was hoping that we will also be possible to specify lambdas at compile time, which is more efficient because boost::function incurs cost equivalent to a virtual method call (it prohibits inlining and dereferences a pointer). And there is the cost of storing the function pointer too... I am imagining things like constrained_value<int, decltype(_1>0 && ! bind(isnan,_1))>. In my perfect imaginary world this would be a zero-size predicate which is default constructable because it carries no data. Thus the expression doesn't need to be duplicated on construction and is stored entirely in the constrained_value typedef. 0 might have to be construct<mpl::integral_c<0> >() :-D I'm sure there are 1000 things wrong with this, just fantasizing... I guess this also falls into the bin of "what's the perfect predicate library?"