[C++ Patch] PR 70540 ("[4.9/5/6 Regression] ICE on invalid code in cxx_incomplete_type_diagnostic...")

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

Hi,

in this regression we ICE during error recovery after an additional 
redundant error message. I think it's one of those cases (we have got 
quite a few elsewhere, in semantics.c too) when it's better to 
immediately return error_mark_node when mark_used returns false, even if 
we aren't in a SFINAE context. To be sure, I double checked that in 
those cases mark_used certainly issues an error, thus we aren't risking 
creating accepts-invalid bugs, it's only matter of fine tuning error 
recovery. Tested x86_64-linux.

Thanks,
Paolo.

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