[phobos] The @property experiment

[Michel Fortin]

> I don't know what dmd fixes you did, but I believe there are several bugs 
> relating to @property which make it so that some things with properties would 
> become impossible. One issue which is going to require either a change in dmd 
> or Phobos is http://d.puremagic.com/issues/show_bug.cgi?id=5707 . There are 
> several functions in std.file which make perfect sense as properties when they 
> take strings but don't make as much sense as properties when they take an int 
> (and _can't_ be properties without UFCS). Functions such as isFile and isDir 
> either take a string which holds a path or a uint which holds the attributes 
> of a file. The compiler currently enforces that a property function is a 
> property function for all of its overloads and that doesn't work in this case. 
> I don't know why that restriction is in place (possibly because the compiler 
> deals with property syntax _before overload resolution?), but it's a problem 
> for std.file which makes it so that either those functions can't be 
> properties, the attribute version must be renamed, or dmd has to be fixed.

You can't overload properties and non-properties today; changing whether @property is enforced or not doesn't affect that. Fixing this problem is entirely orthogonal. Whether it can be fixed is unclear however considering that a property can return a delegate or another callable type which would make things ambiguous.


> I also recall there being a bug which required property functions which 
> returned a ref to be called with parens in at least some cases, though I don't 
> know what bug # it would be.
> 
> So, there are definitely some changes that need to be made in dmd before 
> @property can be enforced, even if Phobos or druntime used them correctly 
> (which it can't in some cases). So, if you fixed them, that's great. If not, 
> whoever keeps on with the work still has quite a bit of work to do.
> 
> Regardless, the fact that we use properties as much as we do in Phobos should 
> definitely help in making sure dmd works correctly with @property.

I might have fixed a few of those DMD bugs while doing all this. I gave myself the broad goal to "make @property work". I made it an error in DMD to call functions with the wrong syntax depending on whether they are @properties or not, and then I went to fix all the errors found when compiling Druntime and Phobos and their unit tests. Whenever I encountered something blocking, I fixed the problem at the source, in the compiler.

These two commits fix things related to @properties and auto returns:

Fixed an issue where @property was lost for functions with deduced return type. 
<https://github.com/michelf/dmd/commit/2ae23a0de72e56c6ec6ba9cd96de16e0140aa73d>

Fixed auto return function with postfix attributes. 
<https://github.com/michelf/dmd/commit/247a40a07a153768bd5c5f2694c3814496aec5e3>

And with this working I went on.

If someone wants to cherry-pick these two commits and apply them to DMD immediately, it might be a good idea.

In fact, now that I think about it, all the DMD commits could probably be merged given that property enforcement is optional (requires the -property command line switch).

-- 
Michel Fortin
michel.fortin at michelf.com
http://michelf.com/