Why does `int ;` compile fine in C, but not in C++?

Question

Consider the following program (see live demo here).

#include 
int main(void)
{
      int ;  // Missing variable name
      puts(\"Surprise\")         


        

Answer 1

The C standard says

A declaration other than a static_assert declaration shall declare at least a declarator (other than the parameters of a function or the members of a structure or union), a tag, or the members of an enumeration.

C++ says

In a simple-declaration, the optional init-declarator-list can be omitted only when declaring a class (Clause 9) or enumeration.

A violation of this in either language requires a diagnostic. The standards do not talk about compiler errors or warnings. A warning is a diagnostic.