Integer precision of function return types

[thinkunix]

Per Nordlöw via Digitalmars-d-learn wrote:
> Should a function like
> 
> ```d
> uint parseHex(in char ch) pure nothrow @safe @nogc {
>      switch (ch) {
>      case '0': .. case '9':
>          return ch - '0';
>      case 'a': .. case 'f':
>          return 10 + ch - 'a';
>      case 'A': .. case 'F':
>          return 10 + ch - 'A';
>      default:
>          assert(0, "Non-hexadecimal character");
>      }
> }
> ```
> 
> instead return an ubyte?

What about using 'auto' as the return type?
I tried it and it seemed to work OK.

Wondering if there are any good reasons to use auto,
or bad reasons why not to use auto here?