I pushed the first wave of this to 10.5, to fix most -Wconversion that was reported for cmake --build . --target innobase code by CMAKE_CXX_COMPILER=clang++-10. This excludes the following:

• code included from include/ or sql/ directories
• generated Bison or Flex code for the internal parsers in InnoDB
• the type mismatch for the second argument of MDL_context::acquire_lock(), global_system_variables.lock_wait_timeout

As part of this, the type of innodb_purge_threads was changed to INT UNSIGNED on all platforms and the documentation string was adjusted to the following:

Number of tasks for purging transaction history

I do not intend to close this ticket before enabling -Wconversion for both clang and gcc.

I got a working build on GCC 10.0.1 and clang 10.0.0, and pushed it to buildbot for wider validation.

It turns out that GCC 10 requires far fewer type casts to silence -Wconversion than its precursors. GCC 5 is by far the worst; I ended up disabling -Wconversion with GCC pragmas for some sections of code where I was not able to suppress the warnings. With GCC 6 and later, everything worked rather reasonably. clang does not seem to warn about potential loss of data when assigning to a bit-field whose size is not 8, 16, or 32 bytes, but GCC does that, and starting with GCC 6 the warning can be suppressed by adding bitwise AND with a constant where all the bits of the bitfield are set.

Follow-on to do this globally: https://jira.mariadb.org/browse/MDEV-26896