The requirements for matching of template template parameters and template template arguments given in 13.4.4 [temp.arg.template] do not mention constraints, leaving questions about whether examples like the following are well-formed:

    auto concept C<class T> {};

    template <template <C T> class U, C V> struct A{};

    template <class T> struct X {};

    A<X,int> ax;  // Well-formed?

    template <template <class T> class U, C V> struct B{};

    template <C T> struct Y {};

    B<Y,int> by;  // Well-formed?

    template <template <class T> class U> struct D{};

    template <C T> struct Z {};

    D<Z> dz;  // Well-formed?

(See also issue 848.)