[C++ RFC / Patch] Another recursive template instantiation issue (c++/51488, etc)

[Paolo Carlini <paolo dot carlini at oracle dot com>]

Hi,

besides the issue discussed a bit some time ago, we have got in Bugzilla 
a number of other issues, all essentially dups of each other (modulo 
irrelevant details, AFAICS)

    c++/51488
    c++/53618
    c++/58059
    c++/56163 (this is two bugs, the second one is dup of c++/55843)

In these the infinite recursion involves instantiate_class_template_1 
but via most_specialized_class (c++/51488 has a backtrace) thus at the 
beginning of the function, and we don't catch it.

To explain *in practice* what I mean, the attached patchlet p avoid all 
these crashes (+ passes the testsuite as-is modulo an XPASS for line 8 
of template/recurse.C).

Unfortunately, at the time I didn't follow in detail the implementation 
of the push_tinst_level mechanism, now I see it used many times with 
little variations, thus I definitely need further guidance (my patchlet 
is loosely inspired by the use in maybe_instantiate_noexcept). In 
particular, I fear memory leaks or mismanagement (I see a ggc_free 
(tinst); in instantiate_alias_template which I don't fully understand).

Also, I wonder if we could provide more detailed information...

Thanks in advance!
Paolo.

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