Returns true if this result is successful and its value compares equal to the value in the success type sugar. Comparison is done using operator== on value_type and A. If A is void, this call aliases bool has_value() const noexcept .

Requires: operator== must be a valid expression between value_type and A, or A is void. If value_type is void, then so must be A.

Complexity: Whatever the underlying operator== has. Constexpr and noexcept of underlying operations is propagated.

Guarantees: None.