functors with template lambdas

[Vlad Levenfeld via Digitalmars-d-learn]

I think this code should be allowed, but it isn't:

	struct Functor (T)
	{
		T a;

		auto ref fmap (alias f)()
		{
			return Functor (f(a));
		}
	}

	auto ref identity (T)(auto ref T a)
	{
		return a;
	}

	void main()
	{
		Functor!int a;

		static auto id (T)(T x)
		{return x;}

		a.fmap!identity; // ok

		a.fmap!id; // ok

		a.fmap!((int x) => x); // ok

		a.fmap!(x => x); // Error: template instance fmap!((x) => x) 
cannot use local '__lambda1' as parameter to non-global template 
fmap(alias f)()
	}

This seems like one of those things that doesn't work because of 
some compiler implementation detail rather than a consequence of 
the language rules but I'm not sure. Opinion?