[Issue 21033] New: enhancement: allow assign to field when the shared owing object has been locked already without cast

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21033

          Issue ID: 21033
           Summary: enhancement: allow assign to field when the shared
                    owing object has been locked already without cast
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: mingwu at gmail.com

```
class A {
        SysTime time;
        synchronized setTime(ref SysTime t) {
                time = t;
        }
}

void main() {
        shared A a = new A();
        SysTime time;
        a.setTime(time);
}
```

Same Error: template std.datetime.systime.SysTime.opAssign cannot deduce
function from argument types !()(SysTime) shared, candidates are:
/usr/include/dmd/phobos/std/datetime/systime.d(659,17):        opAssign()(auto
ref const(SysTime) rhs)

However, we have a lock on the owning shared object, still we need cast to make
it compile:
```
      cast()time = t;
```

I know I can make it work by casting, my question is:

we had a lock on the owning shared object already, WHY we still need the cast
to make it compile.

I think if we enhance the compiler on this simple example, it may cover ~90% of
such use case without the need of the ugly cast.

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