gistfile1.cpp

#include <type_traits>

template< class ConditionType >
class Not
{
    ConditionType m_condition;
public:
    using this_type = Not<ConditionType>;

    template< class ...Args >
    Not( Args&&... args )
        : m_condition( std::forward<Args>(args)... ) {}
    
    Not( const Not& ) = default;
    Not& operator=( const Not& ) = default;

    template< class ...Args >
    bool operator()( Args&&... args ) const
    {
        return ! m_condition( std::forward<Args>(args)... );
    }

    friend inline bool operator==( const Not& left, const Not& right )
    {
        return left.m_condition == right.m_condition;
    }

    friend inline bool operator<( const Not& left, const Not& right )
    {
        return left.m_condition < right.m_condition;
    }

};

class ConditionThatCompile
{
    int m_key;
public:
    ConditionThatCompile( int key ) : m_key( key ) {}

    bool operator()( const int& value ) const
    { 
        return m_key > value;
    } 
    friend inline bool operator==( const ConditionThatCompile& left, const ConditionThatCompile& right ) 
    { 
        return left.m_key == right.m_key; 
    } 
    friend inline bool operator<( const ConditionThatCompile& left, const ConditionThatCompile& right ) 
    { 
        return left.m_key < right.m_key;
    } 
};

class ConditionThatDoNotCompile
{
public:
    bool operator()( const int& value ) const
    { 
        return true;
    } 
    friend inline bool operator==( const ConditionThatDoNotCompile& left, const ConditionThatDoNotCompile& right ) 
    { 
        return true; 
    } 
    friend inline bool operator<( const ConditionThatDoNotCompile& left, const ConditionThatDoNotCompile& right ) 
    { 
        return false;
    } 
};

void do_something();

void somewhere()
{
    Not<ConditionThatCompile> compiling_condition { 42 };
    auto other = compiling_condition; //  error C2664: 'ConditionThatCompile::ConditionThatCompile(const ConditionThatCompile &)' : cannot convert argument 1 from 'Not<ConditionThatCompile>' to 'int'
    if( compiling_condition( 100 ) ) 
        do_something();

    //Not<ConditionThatDoNotCompile> not_compiling_condition;
};