Adding pure to a function type

[Andrei Alexandrescu via Digitalmars-d]

So I'm trying to add "pure" to a function of which type is otherwise 
deduced by the compiler:

package @property pure
T lazilyInitializedConstant(T, alias outOfBandValue, alias initializer)()
if (is(Unqual!T : T))
{
    static T impl()
    {
        ...
    }
    mixin("alias Fun = " ~ typeof(&impl).stringof ~ " pure;");
    return (cast(Fun) &impl)();
}
The type of impl depends on T, and when converted to string is something 
like e.g. "immutable(ulong) function() nothrow @nogc @system".

I tried "alias Fun = pure typeof(&impl);" which compiled (!), but didn't 
work - Fun is not actually a pure type.

So I went with the hammer that will fix anything - a string mixin. Ideas 
for a nicer solution?


Andrei