Elegant way to walk a tree at compile time - that doesn't work

gistfile1.d

import std.typetuple;

template staticReduce(alias F,T...) {
	static if(T.length==0) {
		alias TypeTuple!() staticReduce;
	} else static if(T.length==1) {
		enum staticReduce=T[0];
	} else {
	    enum staticReduce=staticReduce!(F,TypeTuple!(F!(T[0],T[1]),T[2..$]));
	}
}

template concan(string s1,string s2) {enum concan = s1 ~ s2;};
static assert(staticReduce!(concan,"Hello", " world")=="Hello world"); //passes


template Flatten(Tree,string s = "") {
	alias staticReduce!(concan,staticMap!(Flatten,Tree.ChildrenTuple)) Flatten;
}

struct TreeNode(Children...) {
	alias Children ChildrenTuple;
}

alias TreeNode!("A",TreeNode!("B","C","D"),"E") StaticTree;
static assert(Flatten!(StaticTree) == "ABCDE");
//Error: alias static_walk_tree.Flatten!(TreeNode!("A",TreeNode!("B","C","D"),"E")).Flatten recursive alias declaration