How to create a deep copy of an associative array from within a pure function?

[dnspies]

I can't figure out how to copy an associative array from inside a 
pure function.

import std.stdio;
import std.variant;

void main() {
	immutable Variant[string] nested_map = 
["This":mv(["is":mv(),"a":mv()]),
								  "nested":mv(["map":mv()])];
	Variant[string] res_map = nested_map.deep_dup;
	writeln(res_map);
}

@safe
pure Variant[K] deep_dup(K)(const Variant[K] map) {
	Variant[K] res;
	foreach(const K key, const Variant value; map) {
		const Variant[K] under_value = value.get!(const Variant[K]);
		res[key] = under_value.deep_dup;
	}
	return res;
}

@safe
pure Variant mv() {
	Variant[string] inner;
	return Variant(inner);
}

@safe
pure Variant mv(T)(T obj) {
	return Variant(obj);
}

generates a compiler error:

source/app.d(15): Error: pure function 
'app.deep_dup!string.deep_dup.__foreachbody2' cannot call impure 
function 
'std.variant.VariantN!(24u).VariantN.get!(const(VariantN!(24u)[string])).get'
source/app.d(16): Error: pure nested function '__foreachbody2' 
cannot access mutable data 'res'
source/app.d(16): Error: pure nested function '__foreachbody2' 
cannot access mutable data 'res'
source/app.d(16): Error: pure nested function '__foreachbody2' 
cannot access mutable data 'res'
source/app.d(7): Error: template instance app.deep_dup!string 
error instantiating