[C++ Patch] ("[7/8 Regression] ICE with failed class template argument deduction because of invalid template parameter")

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

Hi,

the reason why we ICE here is very simple: we pass an error_mark_node 
generated in cp_parser_template_parameter_list to rewrite_template_parm 
which ICEs (via get_template_parm_index). I believe it makes sense to 
robustify the latter like all the other functions involved in 
build_deduction_guide. The next, error recovery quality, issue is 
whether or not we still want to produce diagnostic about class template 
argument deduction failing: today I noticed - somewhat surprisingly - 
that some compilers still do that (however the diagnostic is more terse: 
eg, one additional error and two notes, not two errors and five notes!). 
I had tested the below which suppresses those additional errors by 
returning error_mark_node from build_deduction_guide if one of those 
parsing-time error_mark_nodes is encountered in the 
build_deduction_guide loop - and lays out a bit of infrastructure.

Tested x86_64-linux.

Thanks, Paolo.

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