This error was frustrating to fix but turned out it was a teensy tiny error of my own.

Context

I was attempting to send a struct over a radio on Arduino

struct State {
    int x;
    int y;
};

struct State MyState;

....

radio.send((uint8_t *)&MyState, sizeof(struct MyState));

So I tried to get the size of an instance instead of the struct itself. No wonder Arduino complained that it couldn't find the struct with the error message of: invalid application ... to incomplete type 'loop()::MyState'.

Also note that & which points to the data, is required to convert the struct into a uint8_t type array for sending. Without it, MyState is a typedef of struct State, and the compiler wouldn't know how to convert a State into a uint8_t.

Fix

Convert sizeof(struct MyState) to sizeof(struct State).

Post Note

Note that the image used in this post has nothing to do with the error.

References

[1] https://arduino.stackexchange.com/questions/26784/is-possible-to-concatenate-integers
[2] https://stackoverflow.com/questions/8915230/invalid-application-of-sizeof-to-incomplete-type-with-a-struct