Verbose checking of range category

[Lutger Blijdestijn]

It would be a welcome improvement. One downside is that the relation between 
the mismatched template and the range check is not there. I worry that it 
will be hard to understand when facing a lot of errors, but will have to try 
to find out. The error message dmd produces in the face of ambiguous 
overloading is the most model (foo matches both bar and baz).

Template constraints are great, but have taken away the possibility of very 
good custom error messages that static if / else / assert have. unaryFun and 
binaryFun of std.functional for example give really helpful messages. I'm 
hesitant to suggest another feature, but maybe an 'else' counterpart to 
template constraint checks could go a long way.