Invariants are useless the way they are defined

[Paolo Invernizzi]

On Sunday, 25 August 2013 at 23:02:29 UTC, Andrei Alexandrescu 
wrote:
> On 8/25/13 2:56 PM, Jonathan M Davis wrote:
>> On Sunday, August 25, 2013 14:16:28 deadalnix wrote:
>>> As they are defined now, invariant are plain useless. I find
>>> myself disabling them one by one in my code as soon as cases 
>>> get
>>> outside simple trivia.
>>>
>>> The problem is that invariant are checked at the 
>>> beginning/end on
>>> public function calls. As a consequence, it is impossible to 
>>> use
>>> any public method in an invariant.
>>>
>>> For instance, just right now I did refactor a struct to use
>>> bitfields, in order to make it more compact. Now I have to 
>>> remove
>>> the invariant as field access are now function calls.
>>>
>>> Another typical situation is when you want to assert certain
>>> properties in a class hierarchy, where calling virtual method 
>>> is
>>> part of the invariant check.
>>>
>>> invariant should (and must to be useful) be inserted at callee
>>> point, when the callee isn't itself subject to invariant
>>> insertion, and around public memeber manipulation (when the
>>> manipulator isn't subject to invariant insertion).
>>>
>>> Any other pattern it doomed to create infinite recursion for 
>>> non
>>> trivial invariant checks.
>>
>> What drives me nuts is the fact that it gets checked when 
>> opAssign is called
>> (rather than just when exiting it). That makes it impossible 
>> to have a member
>> in an invalid state (either through an init which has an 
>> invalid state similar
>> to how floating point values due or because the object hasn't 
>> been initialized
>> yet) and have an invariant. At present, emplace will blow up 
>> in your face if
>> you try and have an invariant with such types.
>
> emplace() should not call assignment for elaborate types.
>
> Andrei

A pull for that is ready, and need review...
Just by coincidence I've hit this problem with emplace and 
invariant and assignment just yesterday, and talked with monarch 
dodra about the very same issue.

https://github.com/D-Programming-Language/phobos/pull/1082

- Paolo Invernizzi