How do I restrict a template class to certain built-in types?

In your first example, static_assert should take a second parameter which would be a string literal, otherwise it’s deemed to fail (edit: dropping the the second parameter is legal since C++17). And this second argument cannot be defaulted.

Your second example is incorrect for several reasons:

• decltype is meant to be used on an expression, not on a type.
• You simply cannot compare types with ==, the correct way to do this is what you try in your first attempt with std::is_same.

So, the right way to do what you are trying to achieve is:

#include <type_traits>

template <typename RealType>
class A
{
  static_assert(std::is_same<RealType, double>::value || std::is_same<RealType, float>::value,
                "some meaningful error message");
};

Moreover, I bet you are trying to constrict your template to floating points values. In order to do this, you can use the trait std::is_floating_point:

#include <type_traits>

template <typename RealType>
class A
{
  static_assert(std::is_floating_point<RealType>::value,
                "class A can only be instantiated with floating point types");
};

And as a bonus, take this online example.