Bug 5123 – Cannot assign null to a class with 'alias this'

If class has 'alias this' for a struct member, attempt to assign null to a class instance fails: ------ struct Foo {} class Bar { Foo foo_; alias foo_ this; } void main() { Bar a; a = null; } ------ aliasthis1.d(8): Error: cannot implicitly convert expression (null) of type void* to Foo

Here alias this is doing its work of "let's pretend" a Bar instance is a Foo instance. To solve this problem you may need something more refined than alias this, something that allows a partial aliasing. I don't think it's an easy thing to do. A related program shows a different outcome: struct Foo {} class Bar { Foo foo_; alias foo_ this; } void main() { Bar a; //a = null; Foo f; a = f; // Access violation! }

> A related program shows a different outcome: > Bar a; > Foo f; > a = f; // Access violation! This is expected. 'a' is an uninitialized, and attempt to assign to a Foo part of null instance causes access violation. This is no different from Bar a; a.foo_ = Foo(); "Partial" assignment of Foo to a valid Bar instance is fine and works as expected: ----- struct Foo { int x; } class Bar { Foo foo_; alias foo_ this; int y; } void main() { Bar a = new Bar; a.x = 1; a.y = 42; assert( a.foo_.x == 1 && a.y == 42 ); a = Foo( 2 ); assert( a.foo_.x == 2 && a.y == 42 ); } ----- My problem is only with special treatment of null: I want to nullify local reference to a Bar instance, not change Bar or Foo inside it.

(In reply to comment #2) > This is expected. 'a' is an uninitialized, and attempt to assign to a Foo part > of null instance causes access violation. This is no different from > > Bar a; > a.foo_ = Foo(); Right, sorry for the noise.

This issue was already fixed by bug 2943. And this was related to bug 6630.