This is D2 code:


Foo!(TArray) foo(TArray)(TArray r) { // the usual helper function
    return Foo!(TArray)(r);
}
struct Foo(TArray) {
    TArray arr;
    bool empty() { return !arr.length; }
    void popFront() { arr = arr[1 .. $]; }
    // int front() { return arr[0]; } // OK
    auto front() { return arr[0]; } // line 12
}
void main() {
    foreach (el; foo([10])) {}
}


DMD 2.043 prints:
test.d(12): Error: forward reference to inferred return type of function call __r1.front()