[Bug c/108054] C2X auto with struct defined in statement expression

[joseph at codesourcery dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108054

--- Comment #2 from joseph at codesourcery dot com <joseph at codesourcery dot com> ---
The basic principle is that auto declarations should always be writable in 
a form without auto, so they should never result in a type escaping to a 
larger scope (but a rule expressed in that form would be very complicated 
to check, as discussed in the context of earlier drafts of the auto 
proposal with a rule expressed in such terms, hence the current version).  
Thus examples like these, where the type declared in a statement 
expression does escape via the type of that expression, are entirely 
appropriately rejected.

For cases of non-ordinary identifiers declared in code using only standard 
constructs (not all of which are properly detected by GCC at present), and 
related issues for some cases other than auto, I provided BSI with a 
document c2x-declaration-context.pdf to include when submitting the NB 
comments I provided on the C2X CD to ISO, which gives 16 examples of code 
using such corner cases and discusses what general principles might be 
consistent with the interpretations previously applied by WG14 in some of 
those cases.  So hopefully that will be included with the CD ballot 
results, if BSI includes that document as requested.