null_check_on_nullable_type_parameter

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

The analyzer produces this diagnostic when a null check operator is used on a variable whose type is T?, where T is a type parameter that allows the type argument to be nullable (either has no bound or has a bound that is nullable).

Given a generic type parameter T which has a nullable bound, it is very easy to introduce erroneous null checks when working with a variable of type T?. Specifically, it is not uncommon to have T? x; and want to assert that x has been set to a valid value of type T. A common mistake is to do so using x!. This is almost always incorrect, because if T is a nullable type, x may validly hold null as a value of type T.

The following code produces this diagnostic because t has the type T? and T allows the type argument to be nullable (because it has no extends clause):

T f<T>(T? t) => t!;

Use the type parameter to cast the variable:

T f<T>(T? t) => t as T;