std::experimental::filesystem missing exception specification noexcept build error regression (libstdc++-11-dev 11.3.0-1ubuntu1~22.04 upgrade)

[Andreas Mohr]

Summary:
On Ubuntu 22.04.1 LTS using clang 11, I'm newly (REGRESSION) getting build errors on std::experimental::filesystem consumption cases:

.
.
.
In file included from ............./../../Compiler/StdFileSystem.h:37:
In file included from /usr/bin/../lib/gcc/x86_64-linux-gnu/11/../../../../include/c++/11/experimental/filesystem:38:
/usr/bin/../lib/gcc/x86_64-linux-gnu/11/../../../../include/c++/11/experimental/bits/fs_path.h:1209:9: error: 'begin' is missing exception specification 'noexcept'
  path::begin() const

This very likely has been introduced by (dpkg.log)
2022-10-27 13:18:37 upgrade libstdc++-11-dev:amd64 11.2.0-19ubuntu1 11.3.0-1ubuntu1~22.04

Details:
No issues seen (working build) until very recently.
Analysis:
extraction of
libstdc++-11-dev_11.2.0-19ubuntu1_amd64.deb
( http://security.ubuntu.com/ubuntu/pool/main/g/gcc-11/ ), where
usr/include/c++/11/experimental/bits/fs_path.h
has:
    // iterators
    class iterator;
    typedef iterator const_iterator;

    iterator begin() const;
.
.
.
  inline path::iterator
  path::begin() const
  {
    if (_M_type == _Type::_Multi)
      return iterator(this, _M_cmpts.begin());
    return iterator(this, false);
  }

, as opposed to 11.3.0-1ubuntu1~22.04:

    // iterators
    class iterator;
    typedef iterator const_iterator;

    iterator begin() const noexcept;
.
.
.
  inline path::iterator
  path::begin() const
  {
    if (_M_type == _Type::_Multi)
      return iterator(this, _M_cmpts.begin());
    return iterator(this, false);
  }

(witness "noexcept" specification newly added, but
INCONSISTENTLY:
at declaration location only, and missing at implementation site)

Apparently fixed by
git://gcc.gnu.org/git/gcc.git commit
42eff613d0c10f "libstdc++: Add 'noexcept' to path::iterator members"
(but currently published for gcc >= 12 release tags only!).

A workaround might be
applying
-Wno-missing-exception-spec
( https://clang.llvm.org/docs/DiagnosticsReference.html )
in all std::experimental::filesystem consumption cases (only!).
Or being forced to
instead actively (source / config modification) switch to
std::filesystem (which
does not seem to have this INCONSISTENCY bug, in
/usr/include/c++/11/bits/fs_path.h
).

$ lsb_release -rd
Description: Ubuntu 22.04.1 LTS
Release: 22.04

$ apt-cache policy libstdc++-11-dev
libstdc++-11-dev:
  Installed: 11.3.0-1ubuntu1~22.04
  Candidate: 11.3.0-1ubuntu1~22.04
  Version table:
 *** 11.3.0-1ubuntu1~22.04 500
        500 http://de.archive.ubuntu.com/ubuntu jammy-updates/main amd64 Packages
        500 http://security.ubuntu.com/ubuntu jammy-security/main amd64 Packages
        100 /var/lib/dpkg/status
     11.2.0-19ubuntu1 500
        500 http://de.archive.ubuntu.com/ubuntu jammy/main amd64 Packages

Thank you for all your awesome service efforts!

P.S.: I got
"In what package did you find this bug?"
"libstdc++-11-dev" does not exist in Ubuntu. Please choose a different package. If you're unsure, please select "I don't know"
( at https://launchpad.net/ubuntu/+source/gcc-11/+filebug )
Though libstdc++-11-dev clearly *is* a "package".
--> this process is overly confusing, thus
possibly should be corrected.

[Adam (justproger)]

Solve this problem, please!!!!!!!!

[Andreas Mohr (andi)]

Yeah indeed, that would be useful. Ahem.

Especially since this bug has severity REGRESSION (something that used to "work appreciably" now doesn't "work appreciably"), as opposed to severity normal (e.g. a feature that is not working yet since it simply is not implemented yet, etc.).
Plus this is - by its nature - base infrastructure stuff, thus rather important...

And the fix ought to be really, really simple (very limited scope: a couple one-liner hunks, to get this CONSISTENCY bug fixed - well, maybe other than potential testing effort).

(myself I cannot provide a patch currently since
I don't have these things running ATM - would need some larger updating effort)

Thank you!

[Matthias Klose (doko)]

please could you provide a test case?

[Launchpad Janitor (janitor)]

[Expired for gcc-11 (Ubuntu) because there has been no activity for 60 days.]