Extension methods weren't supported until version 2.6.0, but this code is required to be able to run on earlier versionsDRT-W1612

The analyzer produces this diagnostic when an extension declaration or an extension override is found in code that has an SDK constraint whose lower bound is less than 2.6.0. Using extensions wasn't supported in earlier versions, so this code won't be able to run against earlier versions of the SDK.

Example

Here's an example of a pubspec that defines an SDK constraint with a lower bound of less than 2.6.0:

environment:
 sdk: '>=2.4.0 <2.7.0'

In the package that has that pubspec, code like the following produces this diagnostic:

extension E on String {
  void sayHello() {
    print('Hello $this');
  }
}

Common fixes

If you don't need to support older versions of the SDK, then you can increase the SDK constraint to allow the syntax to be used:

environment:
  sdk: '>=2.6.0 <2.7.0'

If you need to support older versions of the SDK, then rewrite the code to not make use of extensions. The most common way to do this is to rewrite the members of the extension as top-level functions (or methods) that take the value that would have been bound to this as a parameter:

void sayHello(String s) {
  print('Hello $s');
}