const asserts with == null

[goderbauer]

The following code produces the analyzer error "In constant expressions, operands of this operator must be of type 'bool', 'num', 'String' or 'null'. (const_eval_type_bool_num_string)" on the child == null part.

class Foo {
  const Foo({ this.child, this.text }) : assert(child == null || text == null);

  final Widget child;
  final String text;
}

Seems like this should be allowed though since I can easily transform the condition with deMorgan into something equivalent that the analyzer doesn't complain on. The transformed condition is just harder to read/comprehend.

[bwilkerson]

This restriction comes directly from the language spec. I think the spec would need to be updated before we could update analyzer, so I'm marking this as a language issue.

[lrhn]

Seems like a bug/oversight in the specification. I'm sympathetic to fixing it.
There'll likely be a clean-up of the const/potentially const specification for Dart 2, to iron out a few other problematic cases, but I see no reason not to commit to this immediately.

[a14n]

Dup of #30288 ?

[srawlins]

The example no longer reports an error:

$ dartanalyzer --version
dartanalyzer version 2.0.0-dev.63.0
$ cat 31140.dart 
class Foo {
  const Foo({ this.child, this.text }) : assert(child == null || text == null);

  final String child;
  final String text;
}
$ dartanalyzer 31140.dart 
Analyzing 31140.dart...
No issues found!

[Hixie]

If I try to revert our workaround for this and use assert(child == null || text == null), I get:

[error] In constant expressions, operands of this operator must be of type 'bool' (/usr/local/google/home/ianh/dev/flutter/packages/flutter/lib/src/ma\
terial/tabs.dart:64:15)
[error] In constant expressions, operands of this operator must be of type 'bool', 'num', 'String' or 'null' (/usr/local/google/home/ianh/dev/flutter/\
packages/flutter/lib/src/material/tabs.dart:64:31)

[lrhn]

Issue is definitely still here. The problem does not occur in the test above because the constructor is never called with an object which isn't of those types.

[yjbanov]

FWIW this issue affects Hummingbird. We have to comment out some asserts to make the code run in DDC.

[srawlins]

I think analyzer 2.5.0, with the const-update-2018, produces the correct behavior. Using the example above:

class Foo {
  const Foo({ this.child, this.text }) : assert(child == null || text == null);

  final Widget child;
  final String text;
}

class Widget {
  const Widget();
}

void main() {
  const Foo(child: null, text: null);
  const Foo(child: null, text: 'yay');
  const Foo(child: const Widget(), text: null);
  const Foo(child: const Widget(), text: 'yay');
}

The analyzer only reports an error on the last line:

Evaluation of this constant expression throws an exception.

[yjbanov]

We merged into the main Flutter repo and that repo passes the analyzer, so I guess this is fixed.