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Can I use template aliases as template template parameters?
template <template <typename...> class> struct foo {}; template <typename T> using simple_ptr = std::unique_ptr<T>; foo<std::unique_ptr> a; // this doesn't work, std::unique_ptr has two parameters foo<simple_ptr> b; // does this work? 
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A template argument for a template template parameter is the name of a class template. When the compiler tries to find a template to match the template template argument, it only considers primary class templates. (A primary template is the template that is being specialized.)
For example, given a specialization Stack<int>, “int” is a template argument. Instantiation: This is when the compiler generates a regular class, method, or function by substituting each of the template's parameters with a concrete type.
8. Why we use :: template-template parameter? Explanation: It is used to adapt a policy into binary ones.
In C++ this can be achieved using template parameters. A template parameter is a special kind of parameter that can be used to pass a type as argument: just like regular function parameters can be used to pass values to a function, template parameters allow to pass also types to a function.
Yes, it is apparently allowed. According to the latest draft of the upcoming standard I could find, it is stated that
A template-argument for a template template-parameter shall be the name of a class template or an alias template [...].
However, alias templates seems very seldomly supported at the moment, so you might have some trouble making it work with most compilers.
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