Given some coroutine return type task<T>, the following:

task<int> foo( int a ) { co_return 42; }

when compiled with -Wunused-parameter, will not warn that a is unused. I suspect that it is because a is used during setup of the coroutine frame/promise... but I think most users would not expect that to count as "used."

Actual behavior: no warning.
Expected behavior: warning: unused parameter 'a' [-Wunused-parameter] (when that warning is enabled)

Note that GCC does warn in this scenario. In my coroutine-heavy code base I am thus forced to compile with GCC to find all of my unused parameters.