invalid_extension_argument_count

Extension overrides must have exactly one argument: the value of 'this' in the extension method.

The analyzer produces this diagnostic when an extension override doesn't have exactly one argument. The argument is the expression used to compute the value of this within the extension method, so there must be one argument.

The following code produces this diagnostic because there are no arguments:

extension E on String {
  String join(String other) => '$this $other';
}
​
void f() {
  E().join('b');
}

And, the following code produces this diagnostic because there's more than one argument:

extension E on String {
  String join(String other) => '$this $other';
}
​
void f() {
  E('a', 'b').join('c');
}

Provide one argument for the extension override:

extension E on String {
  String join(String other) => '$this $other';
}
​
void f() {
  E('a').join('b');
}