[CFE, language] Different results for a private field overrides if mixin application is moved to a different library

[annagrin]

Not sure if this is a bug or intended behavior. CFE uses symbols for _foo from the current library in main(), which is different from the symbol for B._foo if class B is defined in a separate library.

This came up in fixing a DDC issue #50119, which resulted in a stack overflow in C()._foo if B was defined in a separate library. I am updating DDC to match other compilers' behavior (described below).

//import 'b.dart';

void main() {
  print('c._foo: ${C()._foo}');  // 'A.foo' if class B is defined in a separate library, 'B._foo' otherwise
  print('d._foo: ${D()._foo}');  // 'A.foo' if class B is defined in a separate library, 'B._foo' otherwise
  print('e._foo: ${E()._foo}');  // 'E._foo'

class C = A with B;

class D extends A with B {}

class E extends A with B {
  String? _foo = 'E._foo';
}

class A {
  String? _foo = 'A._foo';
}

// move this to a separate library b.dart to get different results in main
class B {
  String? _foo = 'B._foo';
}

[annagrin]

@leafpetersen I wonder if the behavior above is intended from the language perspective?

[annagrin]

/cc @nshahan

[lrhn]

This does sound like intended behavior.

Library private names, those starting with _, behave differently depending on whether they are in the same library or in another.

Moving something with a private member to another library is equivalent to renaming every private member to something else, to a name which the original library doesn't use at all.
So, moving the B declaration to a different library means that its _foo member no longer has the same name as the _foo members of the current library, and therefore no longer override A._foo.

[annagrin]

Thanks @lrhn for clarification, will close this as intended then.