Potential implicit return of `null` in method with non-nullable return typeDRT-W1242

The analyzer produces this diagnostic when a method or function has a return type that's potentially non-nullable but would implicitly return null if control reached the end of the function.

Examples

The following code produces this diagnostic because the method m has an implicit return of null inserted at the end of the method, but the method is declared to not return null:

class C {
  int m(int t) {
    print(t);
  }
}

The following code produces this diagnostic because the method m has an implicit return of null inserted at the end of the method, but because the class C can be instantiated with a non-nullable type argument, the method is effectively declared to not return null:

class C<T> {
  T m(T t) {
    print(t);
  }
}

Common fixes

If there's a reasonable value that can be returned, then add a return statement at the end of the method:

class C<T> {
  T m(T t) {
    print(t);
    return t;
  }
}

If the method won't reach the implicit return, then add a throw at the end of the method:

class C<T> {
  T m(T t) {
    print(t);
    throw '';
  }
}

If the method intentionally returns null at the end, then add an explicit return of null at the end of the method and change the return type so that it's valid to return null:

class C<T> {
  T? m(T t) {
    print(t);
    return null;
  }
}