Map Purity

[jmh530 via Digitalmars-d-learn]

My understanding of pure is that a function labeled pure can only 
include pure functions. I've been confused by the fact that a 
function calling map (like below) can be labeled pure without any 
problems. The only way I can rationalize it is that map really 
isn't a function, it's (if I'm understanding it correctly) a 
template that creates a template function that calls a struct 
template.

auto test_map(T)(T x) pure
{
	return x.map!(a => a + a);
}

This is related to
http://forum.dlang.org/thread/ppmokalxdgtszzllzorb@forum.dlang.org?page=1
but maybe my question is more basic.