If you write a template like this one
template <typename T> using X = const T; what's the meaning of const T?
To know about it I wrote this program:
std::cout << std::boolalpha << std::is_same_v<X<int&>, int&> << '\n' << std::is_same_v<X<const int*>, const int* const> << '\n' << std::is_same_v<X<const int&>, const int&> << '\n'; and for my surprise all the answers were true.
Why X < int&> is an int& and not a 'const int&'?
I compiled it with gcc 7.3.0 and clang++ 6.0. Same answer.
A reference to const int is one thing. A const reference to an int is nonsense.
Under the rules of C++, when you const qualify a reference, nothing happens.
This makes lots of sense if you are used to east const, and little if you are a west const user.
int const& is a reference to a const int.
int&const is the same as
int& 
Cv-qualified references are ill-formed except when the cv-qualifiers are introduced through the use of a typedef-name ([dcl.typedef], [temp.param]) or decltype-specifier ([dcl.type.simple]), in which case the cv-qualifiers are ignored. [ Example:
typedef int& A; const A aref = 3; // ill-formed; lvalue reference to non-const initialized with rvalueThe type of
arefis “lvalue reference toint”, not “lvalue reference toconst int”. — end example ]
Tis "pointer to int" (int *), then it's reasonable forconst Tto be "const pointer to int" (int *const), rather than "pointer to const int" (const int *).