The Nim error message is not that bad: Error: unhandled exception: /tmp/gintrosalewski/gen.nim(1011, 18) replist.len == 0 Run
So let us visit that line in gintro/tests/gen.nim if (gFunctionInfoGetFlags(mInfo).int and GIFunctionInfoFlags.IS_CONSTRUCTOR.int) != 0: assert replist.len == 0 # ok it's a fact for constructors, so we don't have to generate code for it Run So it seems to be an GTK update which breaks that assert statement. I will see what I can do do about it.