undefined_operator

The operator '{0}' isn't defined for the type '{1}'.

The analyzer produces this diagnostic when a user-definable operator is invoked on an object for which the operator isn't defined.

The following code produces this diagnostic because the class C doesn't define the operator +:

class C {}

C f(C c) => c + 2;

If the operator should be defined for the class, then define it:

class C {
  C operator +(int i) => this;
}

C f(C c) => c + 2;