operator == overload not called for implied 'true'?

Question

I'm subclassing std::optional and need to delete the operator==(bool) and implement my custom operator==(enum) .

To delete the operator, this worked:

constexpr bool operator == ( bool ) noexcept = delete;

Works great for the code below, throwing a "deleted function" compile error

OptionalSubclass<int> ReturnEvens( int i ) { if ( i % 2 == 0 ) return i; return {}; }
 :
 :
auto result = ReturnEvens(42);
if ( result == true )
   std::cout << *result << " is even" << std::endl;

However, the below code, with an implied 'true', compiles and executes

auto result = ReturnEvens(42);
if ( result )
   std::cout << *result << " is even" << std::endl;

Is there a different operator I should be deleting?

Answer 1

std::optional has an operator bool() that allows it to be converted to a bool for evaluation in a condition. You need to delete that operator as well so it cannot be implicitly converted.