SFINAE cast operator, C++ (gcc)

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#include <map>
#include <string>
#include <type_traits>

template <
	class SubClass, 
	typename EnumType, 
	bool fastStringConvert = true
>
class SmartEnum
{
public:
	template <
		typename SFINAEPostponer = EnumType,
		typename = typename std::enable_if<fastStringConvert, void>::type
	>
	explicit operator const std::string&() const 
	{
		auto name = SubClass::names().find((int)value);
		if (name != SubClass::names().end())
		{
			return name->second;
		}
		else
		{
			static const std::string na("n.a.");
			return na;
		}
	}

template <
		typename SFINAEPostponer = EnumType,
		typename = typename std::enable_if<!fastStringConvert, void>::type
	>
	explicit operator const std::string() const
	{
		auto name = SubClass::names().find((int)value);
		if (name != SubClass::names().end()) return name->second;
		else return std::to_string((int)value);
	}

protected:
	typedef const std::map<int, std::string> Names;
	EnumType value;
};

enum class Foo_type : int { a, b, c };

struct Foo : SmartEnum<Foo, Foo_type, true>
{ 
    typedef SmartEnum<Foo, Foo_type, true> Base;

static const Base::Names &names() 
    { 
        static const Base::Names names = { { 0, "a" }, { 1, "b" }, { 2,"c" }}; 
        return names; 
    } 
};

​x
        static const Base::Names names = { { 0, "a" }, { 1, "b" }, { 2,"c" }}; 
 
#include <map>
#include <string>
#include <type_traits>
​
template <
    class SubClass, 
    typename EnumType, 
    bool fastStringConvert = true
>
class SmartEnum
{
public:
    template <
        typename SFINAEPostponer = EnumType,
        typename = typename std::enable_if<fastStringConvert, void>::type
    >
    explicit operator const std::string&() const 
    {
        auto name = SubClass::names().find((int)value);
        if (name != SubClass::names().end())
        {
            return name->second;
        }
        else
        {
            static const std::string na("n.a.");
            return na;
        }
    }
​
    template <
        typename SFINAEPostponer = EnumType,
        typename = typename std::enable_if<!fastStringConvert, void>::type
    >
    explicit operator const std::string() const
    {
        auto name = SubClass::names().find((int)value);

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