[C++ Patch] PR 51448, 53618, 58059

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

Hi,

some time ago I started analyzing the bugs in Bugzilla involving crashes 
for too deep recursive template instantiation and figured out that this 
subset is probably the simplest to tackle 
(https://gcc.gnu.org/ml/gcc-patches/2013-08/msg01348.html). All these 
crashes involve instantiate_class_template_1 *before* the existing 
push_tinst_level check, via most_specialized_class, and I think it makes 
sense to protect it in a way very similar to that used in 
maybe_instantiate_noexcept. Lately I also tried simply moving the 
existing check before most_specialized_class and it mostly worked for 
C++, but regressions showed up in the libstdc++-v3 testsuite (mostly in 
20_util). As regards the testcases, template/recurse.C is tweaked 
because the diagnostic about template instantiation depth exceed is 
exactly the same but isn't duplicated anymore.

Anyway, tested x86_64-linux.

Thanks,
Paolo.

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