Requirement Propagation and CopyConstructible

The concepts proposal contains a feature called “requirement
propagation” (formerly referred to as “constraint propagation”), which
implicitly defines constraints based on the declaration of a
template. If you didn’t know that requirement propagation existed,
don’t worry; it’s a behind-the-scenes feature that reduces the size of
requirements clauses.

There is one aspect of requirement propagation that we would like to
change. As of N2193, [temp.req.prop] paragraph 2 says:

For every type T that appears as an argument or return type in a
function declarator, the requirement std::MoveConstructible<T> is
implicitly added to the requirements clause. [ Example:

    template<EqualityComparable T>
    bool eq(T x, T y); // implicitly adds requirement CopyConstructible<T>

--end example ]

For various reasons, this feature is problematic and has been removed from ConceptGCC. This change will break some “existing” concepts code. For example, the function “negate” below is currently (according to N2193) well-formed:

  auto concept Negatable<typename T> {
    T operator-(T);
  }

  template<Negatable T>
  T negate(const T& x) { return -x; }

ConceptGCC used to compile this. With today’s development version of ConceptGCC, however, we get an error message:

  beep.C: In function 'T negate(const T&)':
  beep.C:6: error: constructor 'T::T(const T&)' is inaccessible

To remedy the error, add a CopyConstructible requirement, so that “negate” can return its value. Formerly, the
compiler would have added this requirement:

  template<Negatable T> requires CopyConstructible<T>
  T negate(const T& x) { return -x; }