Take https://git.qt.io/vivoutil/libunifex-with-qt and  build the https://git.qt.io/vivoutil/libunifex-with-qt/-/tree/main/http_example_stdexec in it.

In moc_httpdownloader.cpp, the following junk appears:

Q_CONSTINIT const QMetaObject exec::exec::exec::exec::exec::exec::HttpDownloader::staticMetaObject = { {

The multiple spewed instances of the exec namespace are all wrong, search&replacing them with whitespace  i.e. nuking them makes the code compile correctly.

HttpDownloader is not in the exec namespace, and that namespace is closed correctly in the headers.