[foldr]

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.

[saagarjha]

Clang accepts it, but GCC does not. I suspect there's a bug here.

[foldr]

It fails because gcc appears not to support _Generic by default, but that's not the important part of the code. If you comment out the printf, or figure out which compiler options to set, gcc will compile it.