User-defined template error messages

[Nicholas Wilson]

On Sunday, 13 January 2019 at 18:54:52 UTC, Andrei Alexandrescu 
wrote:
> We've had this problem for a long time - template doesn't match 
> because of a complex constraint, the error message is 
> unhelpful, and the user doesn't know what to do.
>
> [...]

I've beat you to it: https://github.com/dlang/DIPs/pull/131 and I 
like mine more. Syntactically it is compile-time contract 
programming, with `in` replaced by `if`, easier to describe that 
way. See that PR thread for more discussion.