non-const

Compare that to: Iterator find(T& value) const; This function signature accepts only non-const references (unless T is const).

Your reference may be const, but something else might still have a non-const reference and mutate the object.

There's plenty of times where both sides are non-const and this trick won't save you when that happens.

Nim also allows passing by non-const reference without any indication of such at the call site.

Are you calling a member function from a const or from a non-const pointer?

If you're writing a library, the API should avoid including container details in function parameters, and const and non-const T should be allowed.

You either need to be very strict about creating non-const references to objects, or you need to enforce immutability at the class level.

A non-const argument will be implicitly cast to const by the compiler.

No later code change can accidentally make the const suddenly non-const.

Something being const in C++ never meant that other things can't modify it, and the risk you describe has nothing to do with it being a const reference, as opposed to the non-const kind.

If you declared most of the mutable types const (and the use cases for a non-const unique_ptr are few) you can avoid most of these issues.

>> Nim also allows passing by non-const reference without any indication of such at the call site.

The draft actually said "If an attempt is made to modify an object defined with a const-qualified type through use of an lvalue with non-const-qualified type, the behavior is undefined.".

RVO also makes "returning" values through non-const references not as necessary.

Whenever I need to return a non-const reference/pointer to some inner data (which is not often) I name the method "mutableFoo()" to make clear what you're getting.

I was hoping that "for (const elem: range)" would be added to mean "for (const auto& elem: range)" because I very often find myself within a non-const method having a non-const container but wanting to iterate with no chance of modifying it.