Request: When static assert fails, display original instantiation line

[Don Clugston]

Using the "sudden-death" static asserts, template libraries can now 
generate very high-quality error messages (in contrast to the garbage 
you usually get in C++). There's a problem, though: "sudden-death" 
static asserts is that they only tell you the line where the static 
assert is, but they normally refer to a problem where the template was 
instantiated from.
So we get error messages like:

library.d(375): (num<50), Number is too big

This has been adequate for library developers, but is unusable for 
library users. We don't need a full template trace, just the original 
file and line where the first template instantiation or mixin was done 
from. Something like:

myproject.d(20): caused static assert in:
library.d(375): (num<50) is false, Number is too big

Or:...

library.d(375): (num<50) is false, Number is too big
myproject.d(20): ultimately instantiated from here.

Coupled with Oskar's recent request for a flag to re-enable template 
backtrace (maybe -nogag or -backtrace) for library developers, I think 
this would completely solve the template error message problem.