Getting the error from __traits(compiles, ...)

[Don]

Bill Baxter wrote:
> We can pretty much already use __traits(compiles,{...}) to implement
> static interface checking.
> 
> template checkInterface(T) {
>     enum bool checkInterface =
>     __traits(compiles,
>     {
>        T x;
>        // some code exercising various aspects of the interface
>     });
> }
> 
> The main problem with this is that a static "implements interface"
> check is done like so:
> 
>    static assert(checkInterface!(myType));
> 
> And all it tells you is a single yes or no.  It would help in
> debugging if you could somehow get the reason for the failure.
> 
> So how to do it?  All that comes to mind is something like the evil
> Errno from C.  A global that gets set by the last failure.
> Let's call it __errmsg, but it could be a pragma, or a __traits thing.
>  If you had that then you could write this:
> 
>    assertImplements!(checkInterface!(myType));
> 
> With:
> template assertImplements(bool v)
> {
>        static if (!v) {
>             pragma(msg, __errmsg);
>             static assert(false);
>        }
> }
> 
> There are lots of ways you could provide such a concept-checker, but
> they all require the basic ability to get the reason for the
> __traits(compiles) failure.
> 
> Any other thoughts about how to get the failure info?   This is
> probably the main complaint against __traits(compiles), that there's
> no way to find out what went wrong if the code doesn't compile.  Often
> it can just be a typo.  I know I've spent plenty of time looking at
> static if(__traits(compiles, ...)) checks that weren't working only to
> discover I switched an x for a y somewhere.  Or passed the wrong
> number of arguments.
> 
> --bb

I think you might be asking for:

static try {
    xxx;
}
catch( CompilerError[] errors){
    pragma(msg, "Failure in Frobozz!");
    pragma(msg, errors[0].msg);
}