Created attachment 55791 [details]
So, parts of this paper has been already fixed in r13-3848-g05119c345797bc04c .
The fallthrough stuff is still outstanding I think and I'll have a look.
The largest nightmare is diagnostics on standard attributes which appertain to something that shouldn't allow them, I'm lost there.
I went through the grammar (except for modules) and tried to find all spots where standard allows attribute-specifier-seq and tried to write that into a testcase in the attached patch, for now for deprecated attribute and marked all -pedantic-errors diagnostics. I've added lots of FIXMEs in there where I don't really know. There are
even 4 spots where we don't parse the attributes at all (and clang++ does), on the other side there are 4 spots where we do parse them and clang++ doesn't.
I think to mark this paper fully resolved we'd need to resolve the FIXMEs and copy/adjust the test for all other standard attributes and verify we emit at least some diagnostics with -pedantic-errors where the standard attributes aren't allowed to appertain to.