Strange implicit conversion integers on concatenation

[lithium iodate]

On Monday, 5 November 2018 at 15:36:31 UTC, uranuz wrote:
> Hello to everyone! By mistake I typed some code like the 
> following without using [std.conv: to] and get strange result. 
> I believe that following code shouldn't even compile, but it 
> does and gives non-printable symbol appended at the end of 
> string.
> The same problem is encountered even without [enum]. Just using 
> plain integer value gives the same. Is it a bug or someone 
> realy could rely on this behaviour?

As long as the integral value is statically known to be a valid 
code point and fits into the numerical rangle of the char-type 
(which plain 'char' in this case), automatic conversion is done. 
If you replace the values inside your enum with something bigger 
(> 255) or negative, you will see that the compiler doesn't 
universally allow all such automatic integral->char conversions. 
You can also see this effect when you declare a mutable vs an 
immutable integer and try to append it to a regular string, the 
mutable one will fail. (anything that can be larger than 1114111 
will always fail, as far as I can tell)
Some consider this useful behavior, but it's not uncontroversial.