There seems to be a defect in the type inference. In TFA's programs, using (but not calling) red gets you a value of the wrong type. Adding an explicit callconv(.Async) to red causes you to get this error:

./src/main.zig:42:32: error: expected type 'fn(*u64) void', found 'fn(*u64) callconv(.Async) void'

const fns = [2]@TypeOf(blue) { red, blue };

After that, changing the array to be an array of @TypeOf(red) gets you a working program. That is, it's apparently fine to store blue in a value of red's type and use blue with @asyncCall as if it was red's type at runtime, but trying to store red in a variable of blue's type is a compile error.

here's a gist demonstrating this stuff that a user posted in the Zig Discord: https://gist.github.com/sharpobject/49bd88416e68606d7812d609...