Fix Issue 9975 - pointsTo asserts because of false pointer in union

[monarchdodra]

This fixes issue 9974 in points to, amongst other issues:

unions: Named unions are now correctly taken into account.
classes: These are handle the same as if they were pointers.
alias this: A subtle bug: An alias this could make pointsTo take the wrong path, analyzing the alias, rather than the actual source. The fix is to really check IsA as opposed to "Convertible to". In particular: "isPointer".

Improved on documentation to avoid pitfalls.

Reworked the restraints: It makes 0 sense to pass target by value, so that was banned. Source was expanded to accept by value pointers and classes.

[monarchdodra]

Could I get a review, or feedback on this? There was actually very little code change, and plenty of unittests to validate the new changes.

[dnadlinger]

Is access to non-set union fields even legal under the D spec? IIRC it isn't in C.

[monarchdodra]

AFAIK, it's not illegal, its just implementation defined.

[dnadlinger]

Oh, I thought it was _un_defined behavior. But as I can't find the relevant paragraph in the spec right now, I'll trust you on that (and it's unclear in how this would apply to D anyway).

[monarchdodra]

I just added some documented unittests. They are thorough, but might be overkill?

Also, why is pointsTo in std.exception ? That don't make no sense to me. I'd have though std.typecons.d.

[jmdavis]

I just added some documented unittests. They are thorough, but might be overkill?

Maybe, but I'll merge it.

Also, why is pointsTo in std.exception ? That don't make no sense to me. I'd have though std.typecons.d.

Probably for the same reason that assumeUnique is in std.exception. Whoever created it (probably Andrei) didn't know where else to put it, and there's a decent chance that it was std.contracts at the time (as listed in TDPL) rather than std.exception after we renamed it. Whatever the exact reason is, it's likely historical and doesn't ultimately make sense, but it is what it is at this point unless we think that it's worth breaking code to move it around.