Difference between "can call" and "can compile"

[Paul Backus]

On Tuesday, 8 September 2020 at 20:04:32 UTC, Steven 
Schveighoffer wrote:
> Here's what it says when there is no constraint (and when I 
> actually call fun inside use):
>
> onlineapp.d(9): Error: undefined identifier oops
> onlineapp.d(4): Error: template instance onlineapp.bad!() error 
> instantiating
> onlineapp.d(20):        instantiated from here: use!(bad)
>
> This is what I want it to do. Why can't it just do that? Why do 
> I need to see 10 more or even 5 more lines of irrelevant error 
> messages?

It sounds to me like your problem is that template constraints 
are doing the exact thing they're designed to do: causing errors 
that would otherwise be reported inside a template to be reported 
outside of it.

Re: irrelevant error messages, I think they're unavoidable in the 
general case. What if there are multiple template overloads? Only 
one of them is going to have the error you care about, but the 
compiler has no way to know which one. And likewise with static 
if and static assert statements inside the template body.

The optimal solution to this problem, from a 
programming-language-theory perspective, is a system like Rust's 
traits that allows the compiler to type-check generic code prior 
to instantiation. Since we don't have that, we're going to have 
to settle for something "good enough."