This is great information. 71000 instantiations of isNarrowString!! Definitely need a hash rather than a linear list. I had no idea. You're right, though, about figuring out a way to avoid this. isNarrowString is nothing more than:

template isNarrowString(T)
{
enum isNarrowString = (is(T : const char[]) || is(T : const wchar[])) && !isAggregateType!T;
}

Makes me wonder why isAggregateType is not 71000 instantiations, too?

It short-circuits through &&.

Maybe we can avoid generating a mangled name for a template if it is an eponymous one that evaluates to a manifest constant? This will save a ton of memory.

Yeah. Interestingly most of these instantiations are inside template constraints or static if, maybe we can do something with that.
There's low-lying fruit here.