Concepts lite, and tight error mesages

[bearophile]

The proposed (and already implemented) "Concepts lite" seem equal 
to D template constraint (but constexpr is worse than D CTFE):

http://isocpp.org/blog/2013/02/concepts-lite-constraining-templates-with-predicates-andrew-sutton-bjarne-s


In that page they show an example of error message:

template<Sortable Cont>
void sort(Cont& container);

and when you call it like this:

list<int> lst = ...;   // oops, bidirectional iterators
sort(lst);             // today, results in very long "template 
spew" error message

getting this short and non-cryptic error message:

error: no matching function for call to ‘sort(list<int>&)’
    sort(l);
          ^
note: candidate is:
note: template<Sortable T> void sort(T)
    void sort(T t) { }
         ^
note: template constraints not satisfied because
note:   'T' is not a/an 'Sortable' type [with T = list<int>] since
note:     'declval<T>()[n]' is not valid syntax


Currently D gives a worse error message, but when a D template 
constraint fails maybe it's possible to show a better error 
message that pinpoints what part of the condition has failed.

Bye,
bearophile