struct Host {
	@disable this(this);
}

void main() {
	uint[] arr;
	
	import std.parallelism;
	auto hostmap = taskPool.map!(function Host(uint a) {
		return Host.init;
	})(arr);
}


Gives me:

/Library/D/dmd/src/phobos/std/parallelism.d(1677): Error: cannot interpret (uint[], ulong, Host[]) at compile time
/Library/D/dmd/src/phobos/std/parallelism.d(1677): Error: static assert  __error
/Library/D/dmd/src/phobos/std/parallelism.d(1996):        instantiated from here: amap!(uint[], ulong, Host[])
fail.d(11):        instantiated from here: map!(uint[])

Turns out the @disable this(this); is key in triggering the error.