extension_as_expression

Extension '{0}' can't be used as an expression.

The analyzer produces this diagnostic when the name of an extension is used in an expression other than in an extension override or to qualify an access to a static member of the extension. Because classes define a type, the name of a class can be used to refer to the instance of Type representing the type of the class. Extensions, on the other hand, don't define a type and can't be used as a type literal.

The following code produces this diagnostic because E is an extension:

extension E on int {
  static String m() => '';
}

var x = E;

Replace the name of the extension with a name that can be referenced, such as a static member defined on the extension:

extension E on int {
  static String m() => '';
}

var x = E.m();