PSA: Failed opDispatch`s dont memoize

[monkyyy]

```d
import std;
enum counter=cast(immutable(void)*)[0].ptr;//ct counter, ignore
auto getcount()=>(*(cast(int*)counter));
auto count()=>(*(cast(int*)counter))++;

struct foo{
     void opDispatch(string s)(){
         enum _=count();//increases getcount by 1 each time its 
compiled
         static assert(0);
     }
}
void bar(foo){}
void foobar(foo){}
unittest{
     foo().bar;
     foo().bar;
}
unittest{
     foo().bar;
     foo().foobar;
     getcount.writeln;//4
}
```

`foo().bar` is being parsed and compiled 3 times; my instincts 
say this could be very slow for "builder patterns" and you may 
want to keep that in mind for large code bases that are slow if 
opDispatch (maybe with complex contracts?) is the only way to 
write something