Problem with ReturnType

[bearophile]

After an 1+ hours long bug hunt, I have reduced my code to the following one.
ReturnType below is a reduced version of std.traits.ReturnType:

template ReturnType(T) {
    static if (is(T R == return))
        alias R ReturnType;
}

void bar(T)(T f) { return f(); }

void main() {
    alias ReturnType!( typeof( (int x){ return x; } ) ) TInt; //problem
    bar((int n=1) { return 2; });
}

The output:
test.d(6): Error: expected 1 arguments, not 0
test.d(10): template instance test.bar!(int delegate(int x)) error instantiating

Is this a bug of DMD/Phobos (I am using 1.028)?

Bye,
bearophile