This is a problem with a return of inner defined struct, I don't know if this specific case is already present in Bugzilla. I think others are already present.

DMD 2.048 compiles and runs this code with no errors:

static struct Bar(T) {
    void spam() {
        foo(1);
    }
}
Bar!T foo(T)(T) {
    return Bar!T();
}
void main() {
    foo(1);
}



But if I pull the Bar struct inside foo(), using auto as return type:

auto foo(T)(T) {
    static struct Bar(T) {
        void spam() {
            foo(1);
        }
    }
    return Bar!T();
}
void main() {
    foo(1);
}


I receive at compile-time:
test.d(4): Error: forward reference to inferred return type of function call foo(1)
test.d(7): Error: template instance test.foo!(int).foo.Bar!(int) error instantiating
test.d(10):        instantiated from here: foo!(int)