Private interface method flagged as not referenced

[dowski]

Problem

In Dart 3.7.2, when I declare a private interface method that I implement in some other classes, the Dart analyzer complains that it isn't referenced.

The declaration `_foo` isn't referenced.

Repro

Here's a DartPad that demonstrates the issue. I'll also paste the code here for clarity.

void main() {
  Bar().bar();
}

final class Bar {
  final foo = Foo();
  
  void bar() {
    foo._foo();
  }
}

final class Foo implements _Foo {
  @override
  void _foo() {}
}

abstract interface class _Foo {
  void _foo();
}

Additional Info

When using VS Code, it seems like the analyzer knows that _foo is used elsewhere because I can jump to implementors and callers. I assume that's powered by the analyzer too, so it's odd that this error is present.

[FMorschel]

Also #54374

CC @srawlins @eernstg

[srawlins]

I believe it is flagged because it is unused as a method on _Foo. There is no reason for it to be in the interface contract of _Foo. It can just be declared on Foo.

[dowski]

Ah you're right! If I update the Bar class to use the interface _Foo, then the warning goes away.

final class Bar {
  // No more warning on _Foo._foo now.
  final _Foo foo = Foo();
  
  void bar() {
    foo._foo();
  }
}

I'll close this out - thanks!