Dereferencing a pointer to incomplete type

Let's say we have two structs in a source file:

struct B {
    int x;
};

struct A {
    beta y;
}; 

In the equivalent header file we have these:

typedef B* beta;
typedef A* alpha;

Also, these function prototypes are defined at the header file:

printAplha(alpha);
compare(const beta, const beta);

In the main file, I have included the specific header file and the code looks like this:

alpha one, two;
printAlpha(one);
printAlpha(two);
//everything works fine up to here
compare(one->y, two->y);

At the last line of the code I am getting

main.c:37:20: error: dereferencing pointer to incomplete type
main.c:37:33: error: dereferencing pointer to incomplete type

I know I can use a wrapper function for compare, the arguments of which would be of type alpha (as the arguments of the compare function cannot be changed - it's a recursive one), but I would like to see if there is any other solution, and why is this happening.

Note: The struct definitions have been written into the source file for creating an opaque data type.

Copyright Notice：Content Author:「Chris」，Reproduced under the CC 4.0 BY-SA copyright license with a link to the original source and this disclaimer.
Link to original article：https://stackoverflow.com/questions/15561242/dereferencing-a-pointer-to-incomplete-type