They'd better not convert that to undefined behavior!
First, the specification for malloc() says you can call it with size 0 and it will still return a pointer to zero bytes; this means an array of bytes having size zero is legal.
Second, having the compiler detect when realloc() is called with size zero changes what should be a library call into an intrinsic, whose very presence in the code is determined by the value of its arguments[0]. If you're going to do that you might as well just put exceptions in the language!
[0] Most compilers assume UB "can't happen" and just remove the offending code altogether.
First, the specification for malloc() says you can call it with size 0 and it will still return a pointer to zero bytes; this means an array of bytes having size zero is legal.
Second, having the compiler detect when realloc() is called with size zero changes what should be a library call into an intrinsic, whose very presence in the code is determined by the value of its arguments[0]. If you're going to do that you might as well just put exceptions in the language!
[0] Most compilers assume UB "can't happen" and just remove the offending code altogether.