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#include <limits>
 
class Foo {
public:
    constexpr Foo(const int value) : value(value) {}    
 
    constexpr static Foo UNKNOWN = std::numeric_limits<int>::max();
    constexpr static Foo ANY     = UNKNOWN - 1;
 
    constexpr operator int () const { return value; }
 
private:
    int value;
};
 
int main() {
	return Foo(0);
}

prog.cpp:7:66: error: 'constexpr const Foo Foo::UNKNOWN' has incomplete type
     constexpr static Foo UNKNOWN = std::numeric_limits<int>::max();
                                                                  ^
prog.cpp:8:46: error: 'constexpr const Foo Foo::ANY' has incomplete type
     constexpr static Foo ANY     = UNKNOWN - 1;
                                              ^

Standard output is empty