How do I force Flutter/Dart to require types?

Question

I'm used to relying on the compiler to catch incompatible type mistakes. By default, Dart offers this only if I remember to specify types. If I forget to include types in the code, then I don't get the type checks.

How do I get the Flutter/Dart compilers to error on the indicated lines in the following code? This code compiles just fine despite being full of type errors.

class Foo {
  String foo() {
    return "foo";
  }
}

class Bar {
  String bar() {
    return "bar";
  }
}

f() { // would like a missing return type error here (e.g. void)
  print("returns nothing");
}

void g(x) { // would like a missing parameter type error here...
  print(x.bar); // ...so this isn't a missing property at runtime
}

void h() {
  String a = f(); // would like this to error...
  print("$a");  // ...instead of this printing "null"
  g(Foo()); // would like this to error for incorrect parameter type
}

If there is a way to do this, how do I do it in Visual Studio Code and Intellij/Android Studio, as well as in dart2js?

Answer 1

You can use the analysis_options.yaml file to make Dart stricter.

analyzer:
  strong-mode:
    implicit-casts: false
    implicit-dynamic: false

Put this file in the root of your project.

This will disable the situations where Dart fallback with dynamic and make it a compile error.

It also disable implicit upcasts like followed:

Object a = 42;
String b = a; // unless `implicit-cast` is disabled, this is valid