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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug libstdc++/104881] New: Document libstdc++ ABI evolution for
 experimental features
Date: Fri, 11 Mar 2022 13:37:14 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D104881

            Bug ID: 104881
           Summary: Document libstdc++ ABI evolution for experimental
                    features
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Keywords: documentation
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---

It might be helpful if the API evolution page at
https://gcc.gnu.org/onlinedocs/libstdc++/manual/api.html contained details =
of
breaking changes to experimental features.

For example, gcc-9.1 was the first release with non-experimental C++17 supp=
ort,
and had ABI changes to std::variant compared with previous releases (which =
is
why you shouldn't mix experimental features from different releases).

r8-1649-g705037247447f4 made the copy constructor trivial in some cases, ma=
king
8.1.0 incompatible with 7.x

r9-6369-g669a6fdcb436ae rewrote large parts of std::variant, making 9.1.0
incompatible with 8.x (and r9-7106-g038bc9bfd6dfd9 and=20
r9-6715-gda97b98ad34145 made further changes).

Similarly, gcc-8.1.0 had triviality changes for std::optional (r8-6100), and
maybe again for 9.1.0 (r9-4229).


There needs to be a caveat about the documentation of those changes being
incomplete, so users don't assume that if it's not mentioned there, it's sa=
fe
to use across versions. It's not supported, and the doc should not be taken=
 as
a guarantee of what can be relied on.=