It was possible in c89 with the implicit int
rule but you needed at least a qualifer or a storage class specifier.
auto x = 3; /* allowed in c89, not valid in c99 */
static y = 4; /* allowed in c89, not valid in c99 */
const z = 5; /* allowed in c89 , not valid in c99*/
a; /* not valid in c89, c99 without a prior declaration */
b = 6; /* not valid in c89, c99 without a prior declaration */