Template constraints in D

[Walter Bright]

Bill Baxter wrote:
> Still, anything 
> containing __traits looks like an afterthought.

A bit.

> Also, I'm not sure how it's going to work in C++, but isn't the compiler 
> going to have a hard time reporting errors in that kind of thing? 
> Failure to instantiate could be because of a very minor typo in the big 
> list of constraints.

True, you won't have an indication which failed. But you won't in C++, 
either, because the overload just won't be matched.

> I'm not sure how C++ will deal with that, but I think it's another place 
> where concept maps help.  They introduce a little redundancy.  They give 
> you a place to say "I think type X satisfies concept Y, and here's how". 
>  That gives the compiler a place to say, "No you're wrong, type X does 
> not satisfy concept Y because Foo isn't actually a method of X".  It 
> seems like it should be possible to generate very readable error 
> messages from that.

Because you can overload based on concepts, I don't see how the compiler 
can give an error message saying it failed to match part of a concept.