Using argument matchers when stubbing a method call results in type error

Question

This is my code:

class Foo {
  int bar(int x) => x;
}

class MockFoo extends Mock implements Foo {}

void main() {
  final foo = MockFoo();
  when(foo.bar(any)).thenReturn(42);
  print(foo.bar(7)); // prints 42
}

How come passing in any as an argument matcher gives me this error:

The argument type 'Null' can't be assigned to the parameter type 'int'.dart(argument_type_not_assignable)

Where are we supposed to use any then, if passing it into the method we are trying to stub just gives me an error.

I was expecting the argument matcher to work, as displayed in these docs.

Answer 1

I think those docs were written before null-safety was a thing. You could change your bar method to receive int? instead of int.

  int? bar(int? x) => x;

Or

int bar(int? x) => x ?? 0;