What are non-referenceable types?

Question

I know that void& is invalid. But, are there any other types, that are non-referenceable?

Answer 1

std::add_rvalue_reference won't make reference type for function type with cv-qualifier or ref-qualifier either. e.g.

cout << boolalpha << is_reference_v<add_rvalue_reference_t<void() const>> << endl;   // false
cout << boolalpha << is_reference_v<add_rvalue_reference_t<void() volatile>> << endl;   // false
cout << boolalpha << is_reference_v<add_rvalue_reference_t<void() const volatile>> << endl;   // false
cout << boolalpha << is_reference_v<add_rvalue_reference_t<void() &>> << endl;   // false
cout << boolalpha << is_reference_v<add_rvalue_reference_t<void() &&>> << endl;   // false

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