use_of_void_result

This expression has a type of 'void' so its value can't be used.

The analyzer produces this diagnostic when it finds an expression whose type is void, and the expression is used in a place where a value is expected, such as before a member access or on the right-hand side of an assignment.

The following code produces this diagnostic because f doesn't produce an object on which toString can be invoked:

void f() {}

void g() {
  f().toString();
}

Either rewrite the code so that the expression has a value or rewrite the code so that it doesn't depend on the value.