Amazingly, it does compile. I believe the declaration that looks as though it shouldn't (because T1 isn't defined) compiles as a function pointer declaration, where the first T1 is the name of the variable defined and the second is, in syntactic terms, the name of the function. With the typedef commented out, you can delete the second T1 on that line.

Note that the type of a variable defaults to int in C, so you can write 'const a' instead of 'const int a'. The return type of a function also defaults to int.