Related: The 'lcc-win' C compiler added operator overloading, default function arguments, and function overloading (see "generic functions") [1]. The Plan 9 C compiler introduced several language extensions, some of which, like anonymous structs/unions would eventually be incorporated into the C standard. Present day GCC accepts the -fplan9-extensions flag [2] which enables some nifty features, like automatically converting a struct pointer to an anonymous field for function calls and assignments.

[1] https://lcc-win32.services.net/C-Tutorial.pdf

[2] https://gcc.gnu.org/onlinedocs/gcc/Unnamed-Fields.html