Shafik Yaghmour

They are different in C and C++ and valid. In C++ it will be:

int foo(int&&)

in C it will be

int foo(int)

Obligatory godbolts, For C:

int foo( int and );
int (*funptr)(int) = foo;

and for C++.

int foo( int and );
int (*funptr)(int&&) = foo;

This is due to how alternative tokens(spellings) are handled in C++ and C. In C++ we get them out of the box [lex.digraph]p2:

In all respects of the language, each alternative token behaves the same, respectively, as its primary token, except for its spelling.12 The set of alternative tokens is defined in Table 1.

and Table 1 includes:

Alternative    Primary
  and            &&

but in C we need to include iso646.h see 7.9 Alternative spellings <iso646.h>:

The header defines the following eleven macros (on the left) that expand to the corresponding tokens (on the right):

and        &&
and_eq     &=
bitand     &
bitor      |
compl      ~
not        !
not_eq     !=
or         ||
or_eq      |=
xor        ^
xor_eq     ^=

We can see including that header:

#include <iso646.h>

int foo( int and ){}
int (*funptr)(int) = foo;

breaks the example