Now that's a DIP that could use some love

[Andrei Alexandrescu]

https://github.com/dlang/DIPs/pull/131

In essence:

* Allow templates to accept multiple constraints.
* Each constraint accepts the syntax "if (expr1, expr2)" where expr2 is 
a string describing the requirement.
* The string corresponding to the constraint that fails is printed in 
the error message if no match is found.

Passes the elevator test. I can be done explaining it in 30 seconds to a 
current D programmer. In fact I just did.

Passes the laugh test. I can keep a straight face throughout, with or 
without a face mask.

Solves a long-standing, difficult problem of outputting meaningful, 
useful error messages.

Is small and integrates well with the language (per static assert and 
friends).