The fundamental problem is that when we call clang to generate
bitcode, we might be using configure results from a different
compiler, which might not be fully compatible with the clang we are
using. In practice, clang supports most things other compilers
support, so this has apparently not been a problem in practice.
But commits 4cfce4e62c, 0af05b5dbb, and 59292f7aac have been
struggling to make typeof_unqual work in this situation. Clang added
support in version 19, GCC in version 14, so if you are using, say,
GCC 14 and Clang 16, the compilation with the latter will fail. Such
combinations are not very likely in practice, because GCC 14 and Clang
19 were released within a few months of each other, and so Linux
distributions are likely to have suitable combinations. But some
buildfarm members and some Fedora versions are affected, so this tries
to fix it.
The fully correct solution would be to run a separate set of configure
tests for that clang-for-bitcode, but that would be very difficult to
implement, and probably of limited use in practice. So the workaround
here is that we hardcodedly override the configure result under clang
based on the version number. As long as we only have a few of these
cases, this should be manageable.
Also swap the order of the tests of typeof_unqual: Commit 59292f7aac
tested the underscore variant first, but the reasons for that are now
gone.
Reviewed-by: Jelte Fennema-Nio <postgres@jeltef.nl>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/flat/92f9750f-c7f6-42d8-9a4a-85a3cbe808f3%40eisentraut.org
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