Implement N4051 - Allow typename in a template template parameter

[Ed Smith-Rowland <3dw4rd at verizon dot net>]

This little annoyance has bugged me a couple of times over the years:

template<typename T> struct A {};
template<typename T> using B = int;

template<template<typename> class X> struct C;
C<A> ca; // ok
C<B> cb; // ok, not a class template
template<template<typename> typename X> struct D; // error, cannot use typename here


C++17 it seems allows typename in template template parms.

Built and tested on x86_64-linux.  Allowed with std=gnu/c++1z of course.

OK?

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Attachment: patch_typename_template_template_2
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