False warning of `dead_null_aware_expression` when using map pattern

[jtdLab]

When using this code:

foo.dart

final map <String,dynamic>{}

bar.dart

if (map case {'language': String? language}) {
  language ??= 'en';
}

a dead_null_aware_expression is reported.

[dart-github-bot]

Summary: The dead_null_aware_expression warning is incorrectly triggered when using a map pattern with a nullable value in an if statement. The warning should not be issued as the null-aware operator (??=) handles the potential null value.

[devoncarew]

Thanks for the report! Can you post a full repro, perhaps as a dartpad link?

[jtdLab]

https://dartpad.dev/?id=a2a5897e1125a9dd930716186f9423f8

[julemand101]

@jtdLab
Your example in the dartpad does not match the code you posted in the issue. Your code in DartPad are:

void main() {
  if (someMap() case {'language': String? language}) {
    language ??= 'en';
  }
}

Map<String, String> someMap() {
  return {};
}

Since the signature of someMap() are that it returns Map<String, String> it makes sense you get the error since the value would never be possible to be null since the values of this map can't be null.

If you instead change your someMap() to the one from your initial example:

Map<String, dynamic> someMap() {
  return {};
}

Or:

Map<String, String?> someMap() {
  return {};
}

The error goes away since now it is actually possible for the returned map value to be null.