An extension override can only be used to access instance membersDRT-W1351

The analyzer produces this diagnostic when an extension override is found that isn't being used to access one of the members of the extension. The extension override syntax doesn't have any runtime semantics; it only controls which member is selected at compile time.

Example

The following code produces this diagnostic because E(i) isn't an expression:

extension E on int {
  int get a => 0;
}

void f(int i) {
  print(E(i));
}

Common fixes

If you want to invoke one of the members of the extension, then add the invocation:

extension E on int {
  int get a => 0;
}

void f(int i) {
  print(E(i).a);
}

If you don't want to invoke a member, then unwrap the argument:

extension E on int {
  int get a => 0;
}

void f(int i) {
  print(i);
}