I am familiar with how const behaves for "runtime" code and it makes perfect sense, but since "everything done during compilation is const" (obviously this is not my interpretation, not standard wording) I wonder why C++ cares about if the reference is const or not.
In particular this example does not compile when const is removed:
struct Country {
std::string_view name;
};
template<const /* <-- required */ Country& country>
struct S{
void print() {
fmt::print("{}\n", country.name);
}
};
static constexpr Country sg{.name="Singapore"};
int main() {
S<sg> sg_printer;
sg_printer.print();
}
error for gcc is something like:
error: binding reference of type 'Country&' to 'const Country' discards qualifiers
My only guess that nobody thought about this or that this was done to be same as for "normal" code so that it is consistent.
Non type template arguments that are references can be non-const.
The reason for your error is that you are trying to bind a const object to a non-const reference. The error is gone if both const are removed:
#include <string>
#include <iostream>
struct Country {
std::string name;
};
template<Country& country> // no const necessary
struct S{
void print() {
country = Country{"foo"};
std::cout << country.name;
}
};
static Country sg{"sg"}; // no const
int main() {
S<sg> sg_printer;
sg_printer.print();
}