Here's a sneaky little bug

[Steven Schveighoffer]

On Fri, 24 Jun 2011 14:28:45 -0400, Andrej Mitrovic  
<andrej.mitrovich at gmail.com> wrote:

> import std.stdio;
>
> void main()
> {
>     bool state = false;
>     writeln("state is: " ~ state ? "true" : "false");
> }
>
> writes:
> true
>
> Whoa, what happened? Well, this should explain things:
>
>     bool state = false;
>     auto str = "bla" ~ state;
>
> What (I assume) happens is the state boolean is converted to an int,
> and since chars are ints in disguise and interchangeable you can
> concatenate them with strings.
>
> So the original code acted like it was written like this:
>     bool state = false;
>     writeln(("state is: " ~ state) ? "true" : "false");
>
> And what we wanted was this:
>     bool state = false;
>     writeln("state is: " ~ (state ? "true" : "false"));
>
> Anyway I just wanted to share how forgetting parens can introduce bugs  
> in code.

I can never remember precedence for ?:, so quite often I will  
over-parenthesize those expressions.  I also typically over-parenthesize  
any expressions involving logical operations (i.e. & and |)

Rule of thumb -- minimal parentheses that establish order of precedence,  
even if unnecessary, don't ever hurt :)

But in any case, I'm really surprised "state is: " ~ false works at  
all...  Since when can you append non-character types to strings?  This is  
bound to result in bugs, since many people come from languages where  
appending a non-string type to a string results in a conversion to a  
string.

I'd expect "state is: " ~ false to result in "state is: false" if it  
compiles at all.  This may not be a bug, but it probably warrants an  
enhancement.  D goes through great pains to avoid automatic string  
conversion, I think it should reject any attempts at it, even if they are  
accidental.

-Steve