I’m curious what issues people were running into with Swift’s built in C++ interop? I haven’t had the chance to use it myself, but it seemed reasonable to me at a surface level.

There's a list of unsolved problems in this Ladybird issue, now closed because they dropped Swift: https://github.com/LadybirdBrowser/ladybird/issues/933

for example: "Swift fails to import clang modules with #include <math.h> with libstdc++-15 installed. Workaround: None (!!)"