Don't null check (i.e. `!`) a potentially nullable type parameter.

== Why is this an issue?

Given a type parameter `T`, the type `T?` is nullable version of `T`, i.e. A variable `x` of type `T?` can either hold a value of type `T` or `null`.

However, when `T` itself is bound to a nullable type, a variable `y` of type `T` can hold a `null` value by itself. For example, if `y` is defined as `int?`, `null` is a valid value for `y`.

Therefore, when `T` is bound to a nullable type, `x` can hold `null` as a valid value, which means that `x!` is an incorrect check for the validity of `x`, since it would raise a runtime exception when `x` holds `null`.

[source,dart]
----
void main() {
  int? x = null;
  bangT(x); // Throws a runtime exception
  asT(x); // Returns null
}

T bangT<T>(T x) => x!;
T asT<T>(T x) => x as T;
----

== How to fix it

Use the `as` operator, instead of the `!` operator. `as T` will not raise exception when `x` is `null` and `T` is bound to a nullable type.

=== Code examples

==== Noncompliant code example

[source,dart,diff-id=1,diff-type=noncompliant]
----
void main() {
  getOrDefaultIf<int?>(() => 42, false);
}

T getOrDefaultIf<T>(T retriever(), bool condition) {
  T? result;
  final retrieveIfCondition = () { 
    if (condition) {
        result = retriever();
    }
  };
  
  retrieveIfCondition();
  return result!; // Noncompliant
}
----

==== Compliant solution

[source,dart,diff-id=1,diff-type=compliant]
----
void main() {
  getOrDefaultIf<int?>(() => 42, false);
}

T getOrDefaultIf<T>(T retriever(), bool condition) {
  T? result;
  final retrieveIfCondition = () { 
    if (condition) {
        result = retriever();
    }
  };
  
  retrieveIfCondition();
  return result as T;
}
----

== Resources

=== Documentation

* Dart Docs - https://dart.dev/tools/linter-rules/null_check_on_nullable_type_parameter[Dart Linter rule - null_check_on_nullable_type_parameter]
* Dart Docs - https://dart.dev/null-safety[Language - Null safety]
* Dart Docs - https://dart.dev/null-safety/understanding-null-safety#non-null-assertion-operator[Language - Non-null assertion operator]


ifdef::env-github,rspecator-view[]

'''
== Implementation Specification
(visible only on this page)

=== Message

* The null check operator shouldn't be used on a variable whose type is a potentially nullable type parameter.

=== Highlighting

The `!` operator in `<variable>!`.

'''
== Comments And Links
(visible only on this page)

endif::env-github,rspecator-view[]