Difference in compiletime vs compiletime or compiler bug?

[Sjoerd Nijboer]

I've got a snippet of code which I have narrowed down to the 
following:
'import std.stdio;

enum string[] mixins = ["public bool qux(int i, char c)
             {
                 throw new Exception(\"not implemented\");
                 // Add all arguments to a struct and serialize 
that struct.
             };", "
public bool qux(string s)
             {
                 throw new Exception(\"not implemented\");
                 // Add all arguments to a struct and serialize 
that struct.
             };
			"];

struct C
{
	static foreach (m; mixins)
	{
		mixin mix!m;
	}
}

mixin template mix(string s)
{
	mixin(s);
}

void main()
{
	import std.traits : ReturnType;

	auto c = C();
	pragma(msg, c.qux.mangleof); // writes 'v' to the console
	pragma(msg, ReturnType!(c.qux));
}'

The exact error this returns is as follows:
'Error: template instance std.traits.ReturnType!(qux) does not 
match template declaration ReturnType(func...)
   with func = (qux)
   must satisfy the following constraint:
        isCallable!func
source/app.d(34,2):        while evaluating pragma(msg, 
ReturnType!(qux)
'

As far as I know, this "should" work.
When I delete one of the functions out of the mixin it suddenly 
compiles with a mangled name, but when I place the second 
function in there it won't.
When calling the mixin directly instead of through the template 
mixin it breaks with thesame error message.

I don't know if this an actual compiler bug or if this is me not 
properly understanding D.
I'm using DMD64 D Compiler v2.089.1

Any help is appreciated.