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

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.

Your code is illegal (i.e. erroneous, ill-formed, constraint-violating) in both C and C++. The reason you get a "warning" in one language and "error" in another is just a quirk of your compiler and your compiler setup. After all, neither language really formally differentiates between "warnings" and "errors". GCC under its default settings just happens to be more permissive in C mode (mostly for historical reasons).

Use -pedantic-errors in GCC, and you will get an "error" in C code as well. (Note that -pedantic-errors does not simply blindly turn all "warnings" into "errors". It attempts to report only actual constraint violations as "errors".)

The syntax of declaration is defined as (omitting init-declarator-list and init-declarator):

C11 6.7 Declarations

declaration:    declaration-specifiers init-declarator-list opt ;    static_assert-declarationdeclaration-specifiers:    storage-class-specifier declaration-specifiers opt    type-specifier declaration-specifiers opt    type-qualifier declaration-specifiers opt    function-specifier declaration-specifiers opt    alignment-specifier declaration-specifiers opt

Note that declaration-specifiers is defined recursively, but each with an opt indicates it's optional.

Also, the following clause 6 states:

The declaration specifiers consist of a sequence of specifiers that indicate the linkage,storage duration, and part of the type of the entities that the declarators denote. The initdeclarator-list is a comma-separated sequence of declarators, each of which may have additional type information, or an initializer, or both. The declarators contain the identifiers (if any) being declared.

Note the words if any.