__traits(compiles, ...) returns true for syntactic garbage and for semantically incorrect code

[Simen Kjaeraas]

On 2012-42-17 11:12, Pavel <proger79 at gmail.com> wrote:

> Either I do not understand the work of this feature or it is an obvious  
> bug:
>
> import std.stdio;
> import std.conv;
>
> void main()
> {
>     enum string expr = "DMD compiles this garbage ... iiiii - \" ####  
> $$$";
> 	
>     enum bool bTest = __traits(compiles, expr);
>     enum bool bTest2 = __traits(compiles, "int i = q{};");
> 		
>     writeln("bTest: " ~ to!string(bTest));
>     writeln("bTest2: " ~ to!string(bTest2));
> }
>
> Produces (tested with dmd32 2.060 and dmd32 2.059):
>    bTest: true
>    bTest2: true
>
> (http://dpaste.dzfl.pl/5d338ab3)
>
> Could you please somebody explain this?
>
> Thanks,
> Pavel

Let's start off with the obligatory 'You're doing it wrong'.

__traits(compiles, ...) does not check the content of strings.
If you want that, use mixin("somestring").

What your code does is simply check if the string is one that
could be embedded in D code, which both of them can.

If instead of using strings you do this:

    enum bool bTest2 = __traits(compiles, {int i = q{};});

You will see that the result is false.

-- 
Simen