[PR C++/59930] template friend classes & default args

[Nathan Sidwell]

PR 59930 concerns some problems with templated friend classes (of 
templates).  In rying to clean up our handling, I discovered we were 
accepting default args of such things.  This is ill formed

[temp.param]/12 'A default template-argument shall not be specified in a 
friend class template declaration.'

This patch addresses that problem by extending check_default_tmpl_args 
to deal with such friends.

We'll still have to deal with:
template <typename T> class X {
    template <typename U, T> friend class Y;
};

which is probably just as involved.  But at least this removes one 
source of confusion.

nathan
-- 
Nathan Sidwell
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