Code shared from run.dlang.io.

main.d

void main()
{
    import std.stdio: write, writeln, writef, writefln;
    int i;
    
    enum Foo { E }
    Foo f;
    i = f;           // OK
    f = i;           // error
    f = cast(Foo)i;  // OK
    f = 0;           // error
    f = Foo.E;       // OK
}