Adding a new design constraint to D

[Max Samukha]

On Saturday, 18 June 2022 at 08:04:55 UTC, FeepingCreature wrote:

> a function in a module is "on the same level" as a method in 
> the class.

If it is on the same level, then I would expect 'foo' below to 
lock the mutex and run the invariant check:

```d
synchronized class C
{
     private int x;
     private int y = 1;

     invariant() { assert(y == x + 1); }
}

void foo(C c, int x)
{
     c.x = x;
     c.y = x + 1;
}
```