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The code for sqlite3VdbeMemRelease can be found here: https://www.sqlite.org/src/file?name=src/vdbemem.c&ci=4218c7...

I don't see any type-punning of the flags going on. (The sqlite3VdbeMemRelease method doesn't touch the flags directly, but it calls vdbeMemClear which in turn calls vdbeMemClearExternAndSetNull which resets flags to MEM_Null.)

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That's exactly what I expected. SQLite is one of the very few pieces of software where, when compiled on a new compiler, a bug is much more likely to be in the compiler than in the software it's compiling.

That's not because compilers aren't buggy; compilers have bugs. It's because most software is compiled with very few compilers and has fairly low ratio of effort invested in being correct vs effort invested in writing features. Neither of those are true for SQLite.

Notice that C allows you to introduce UB for sqlite3VdbeMemRelease in any TU of the program, not only the TU that defines it, so you'd need to check every declaration in the program at least, and that would include checking at least every file that includes its header file, making sure that, e.g., all defines are identical, among other things.

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