Better error messages - from reddit

[Andrei Alexandrescu]

On 3/4/19 10:47 AM, H. S. Teoh wrote:
> On Mon, Mar 04, 2019 at 02:50:11PM +0000, Adam D. Ruppe via Digitalmars-d wrote:
> [...]
>> How many template constraint messages would be nice if it just said
>> "missing popFront" instead of 40 lines of overload failed spam?
> [...]
> 
> This is why I'm starting to think sig constraints are not the genius
> idea they first appeared to be.

A while ago, someone in this forum (or Walter or even myself) had an 
idea on how to associate error messages with template constraints. 
Someone pointed out an in-the-works DIP about it. Couldn't find it, 
where is it?