I've launched a nightly test run here - https://github.com/Mytherin/duckdb/actions/runs/13518555521

Turns out the reason this changed is that DUCKDB_API cannot be used in MinGW for inlined functions - which happens in a number of places in the codebase. It seems to me like the correct fix here is to remove DUCKDB_API from those calls instead of disabling DUCKDB_API for the MinGW platform entirely. I'll give that a go.

I was looking at the man page for x86_64-w64-mingw32-g++, I came across this:

   -fno-keep-inline-dllexport
       This is a more fine-grained version of -fkeep-inline-functions, which applies
       only to functions that are declared using the "dllexport" attribute or declspec.

   -fkeep-inline-functions
       In C, emit "static" functions that are declared "inline" into the object file,
       even if the function has been inlined into all of its callers.  This switch does
       not affect functions using the "extern inline" extension in GNU C90.  In C++,
       emit any and all inline functions into the object file.

Maybe adding one of these options to the compile commands of the C-API files with inlined exported functions is also another way to approach this?

After struggling with this for a bit I've solved it in a different manner - by switching the name of the definition in duckdb.h to DUCKDB_C_API