[Issue 18744] New: Class l-values can be implicitly converted to `Object` l-values, even in safe code

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=18744

          Issue ID: 18744
           Summary: Class l-values can be implicitly converted to `Object`
                    l-values, even in safe code
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Keywords: accepts-invalid, safe
          Severity: major
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: code at klickverbot.at

---
@safe:

class A { int* a; }
class B { long b; }

// Either of these shouldn't be allowed:
void boom1(ref A a, B b) { (cast(Object)a) = b; }
void boom2(ref A a, B b) { (true ? a : b) = b; }

int main() {
  A a = new A;
  B b = new B;
  boom1(a, b);
  boom2(a, b);
  return *a.a;
}
---

compiles fine on DMD 2.079, but covariance in l-values can't be allowed. This
is the same problem often illustrated using containers.

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