Very confusing error message when calling a class method from an invariant

[Meta]

On Wednesday, 10 March 2021 at 04:57:19 UTC, Paul Backus wrote:
> On Wednesday, 10 March 2021 at 03:39:15 UTC, Meta wrote:
>> class Human {
>>     static immutable MAX_AGE = 122;
>>
>>     bool alive = true;
>>     int age = 0;
>>     //Error: mutable method onlineapp.Human.checkAge is not 
>> callable using a const object
>>     invariant(checkAge());
> [...]
>>
>> What the hell does this even mean, and where does it come 
>> from? Adding `inout` to `checkAge` actually does cause it to 
>> compile and run too. WTF?
>
> From the language spec [1]:
>
>> The invariant is in the form of a const member function.
>
> So, inside the invariant, the object is treated as const, which 
> means you can't modify it and can only call const methods.
>
> [1] https://dlang.org/spec/class.html#invariants

Now that you mention it, I'm pretty sure I've run into this 
before; I must've forgotten about it. I understand the rationale 
behind this, but it doesn't really make sense IMO that only 
invariants treat the object as const, and not pre/post conditions 
as well. Ah well. Thanks for quick answer.