игнорирование компилятором выражения с несуществующим полем структуры

template <typename T, typename = decltype(T::field)>
return-type name (argument-list) { /* */ }

template <typename T>
return-type name (argument-list) { /* */ }

call of overloaded ‘name()’ is ambiguous

#include <iostream>
#include <type_traits>

template< typename S >
class has_d_theta
	{
	private :
		template< typename P, P > struct checker;

		template< typename D > static std::true_type test(
				checker< decltype(&D::d_theta), &D::d_theta> * );

		template< typename D > static std::false_type test(...);

	public :
		enum { value =
			std::is_same< std::true_type, decltype(test<S>(0)) >::value };
	};

struct data_with_d_theta
	{
		int d_alpha = 0;
		float d_beta = 0;
		double d_theta = 0;
	};

struct data_without_d_theta
	{
		int d_alpha = 0;
		float d_beta = 0;
	};

template< typename S >
typename std::enable_if< has_d_theta<S>::value >::type
show_value( const S & s )
	{
		std::cout << "alpha: " << s.d_alpha
				<< ", beta: " << s.d_beta
				<< ", theta: " << s.d_theta
				<< std::endl;
	}

template< typename S >
typename std::enable_if< !has_d_theta<S>::value >::type
show_value( const S & s )
	{
		std::cout << "alpha: " << s.d_alpha
				<< ", beta: " << s.d_beta
				<< std::endl;
	}

int main()
	{
		show_value( data_with_d_theta{} );
		show_value( data_without_d_theta{} );
	}