add mixin template StandardExceptionConstructor

[jamadagni]

Convenience mixin for trivially subclassing exceptions. Documentation and unittest included.

[jmdavis]

I still think that this is a waste of time, because you the constructors that get mixed in can't be documented, but if you're going to do do this, the correct constructors would be

this(string msg, string file = __FILE__, size_t line = __LINE__, Throwable next = null) @safe pure nothrow
{
    super(msg, file, line, next);
}

this(string msg, Throwable next, string file = __FILE__, size_t line = __LINE__) @safe pure nothrow
{
    super(msg, file, line, next);
}

[jamadagni]

Well the next parameter is not intended to be used from user code as you can see at: http://dlang.org/phobos/object.html#.Exception: "The next parameter is used internally and should always be null when passed by user code", and likewise the second overload is only needed for convenience of providing the next right after the message without having to manually include __FILE__ and __LINE__, so there's no point in including that overload either.

As for documentation, I thought using the mixin would actually enable it if you include a doc-comment block before it? Besides, if all you are doing is a trivial constructor for purposes of identification, then there should be no need for documentation since it's constructed in no way different than a regular exception.

[jmdavis]

Okay. I just tested it, and it looks like if you put a ddoc comment on the constructor, then that ddoc comment will actually show up in the exception's documentation. I did not think that that was the case. So, you're going to need to document the constructor(s) in the mixin.

As for next, every exception in druntime declares a constructor which takes it as do most of the Phobos exceptions. It's used for exception chaining. Please declare the constructors like so

/++
    Params:
        msg  = The message for the exception.
        file = The file where the exception occurred.
        line = The line number where the exception occurred.
        next = The previous exception in the chain of exceptions, if any.
  +/
this(string msg, string file = __FILE__, size_t line = __LINE__, Throwable next = null) @safe pure nothrow
{
    super(msg, file, line, next);
}

/++
    Params:
        msg  = The message for the exception.
        next = The previous exception in the chain of exceptions.
        file = The file where the exception occurred.
        line = The line number where the exception occurred.
  +/
this(string msg, Throwable next, string file = __FILE__, size_t line = __LINE__) @safe pure nothrow
{
    super(msg, file, line, next);
}

That is the standard way to do it, and the only exceptions that don't do that are ones which have alternate constructors, because they're not simply taking a message.

[jmdavis]

Also, this code has no business in object.d. It's a helper template, not something that needs to be pulled into every piece of D code ever. I'd have commented on that before, but I wasn't paying enough attention to where you put it. It should go in std.exception.

[jmdavis]

So, feel free to create a PR with this template mixin in std.exception, but I'm closing this PR, because it doesn't below here.