[C++ Patch/RFC] PR 53184

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

Hi,

per the audit trail, this issue appears to boil down to two separate issues:
- The warning doesn't appear universally useful, thus it would be nice 
to give it a name in order to enable disabling it.
- As shown by the testcase, sometimes the wording is misleading: it 
talks about 'anonymous namespace', where, as clarified by Jason in the 
trail, the issue is really about a type with no linkage, no namespace 
involved.

- The former is easy done, I picked: -Wsubobject-linkage. Makes sense?
- The latter is a little more tricky, because it doesn't seem always 
easy to tell one case from the other, in particular when templates are 
involved (eg, g++.dg/warn/anonymous-namespace-3.C) and the linkage issue 
involves template arguments. Given that the warning doesn't seem 
terribly important (as another data point, clang doesn't have it), so 
far I have conditionals which reliably figure out cases of anonymous 
namespace and cases of no linkage (per the testcase at issue, for 
example) and otherwise fall back to an 'or' wording. I hope the 
improvement is good enough. Alternately, I suppose the warning could use 
a completely different, more generic, wording, but in that case 
testcases like anonymous-namespace-3.C will need adjustment.

Tested x86_64-linux.

Thanks,
Paolo.

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