Error: A library can't opt out of null safety by default, when using sound null safety
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Solution 1
To solve your problem, remove the following line from text_test.dart:
// @dart = 2.8
This annotation is for opting out of sound null safety (see this and this). However, libraries (packages) are not allowed to do that if they want to be sound null safe.
Solution 2
The problem also occurs when you have a mixed-version code i.e. the code is not fully migrated to sound null safety. To run the app:
-
Using IDE
Add
// @dart=2.9
at the top in yourmain.dart
file and run the app using the Play ► icon.// @dart=2.9 import 'package:flutter/material.dart'; void main() { //... }
-
Using command line
flutter run --no-sound-null-safety
or to be specific (say in chrome)
flutter run -d chrome --no-sound-null-safety
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Comments
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Suragch about 2 years
When upgrading a Flutter package for null safety I got this error when running
flutter test
:Error: A library can't opt out of null safety by default, when using sound null safety. // @dart = 2.8 ^^^^^^^^^^^^^^ Failed to load "/Users/xxxxx/test/text_test.dart": Compilation failed
The answer was right there but I was still confused for a while, so I'm adding my answer below.