Help with "cannot alias an expression" compile error

[Jacob Carlborg via Digitalmars-d-learn]

I'm trying to do some form of reflection abstraction, here's my sample code:
import std.meta;

struct Fields(T)
{
    private static alias toField(alias e) = Field!(__traits(identifier, 
e));
    alias fields = staticMap!(toField, T.tupleof);

    static alias map(alias F) = staticMap!(F, fields);
    static alias filter(alias pred) = Filter!(pred, fields);
}

struct Field(string n)
{
    enum name = n;
}

struct Foo
{
    int a;
}

void main()
{
    Foo value;

    enum toName(alias e) = e.name;
    enum pred(alias e) = true;

    alias a = Fields!(Foo).filter!(pred); // works
    alias b = Fields!(Foo).map!(toName); // works

    alias c = Fields!(Foo).init.filter!(pred); // works
    alias d = Fields!(Foo).init.map!(toName); // Error: alias d cannot 
alias an expression Fields().tuple("a")
}

Alias "a" and "b" compiles just fine. Alias "c" compiles as well but not 
alias "d", which I don't understand why. I'm fully aware that it's not 
possible to alias an expression, but what I want is to alias the tuple 
of strings that is returned.

--
/Jacob Carlborg