> memcpy is just another function in some library. It might be libc or not. I don't expect compiler to care about it, just put function call into object code and move on.
The compiler has special knowledge of memcpy and may introduce calls to it even if you don't call it anywhere in your code - see this : https://gcc.godbolt.org/z/_w78qd
Hence standard library function names are "reserved for use as identifiers with external linkage", ie unless your implementation is 'freestanding', you're not allowed to re-define them.
The code above does not do so.
Contrary to what other people have suggested, according to my understanding of the C language standard, the word `memcpy` is not magical in any way, and undefined behaviour only comes in because the code will call `memcpy` with incorrect signature.
The compiler has special knowledge of memcpy and may introduce calls to it even if you don't call it anywhere in your code - see this : https://gcc.godbolt.org/z/_w78qd