If the shift count value is a negative value then both left shift and right shift operations are undefined1:
int x = 5 << -3; /* undefined */ int x = 5 >> -3; /* undefined */
If left shift is performed on a negative value, it's undefined:
int x = -5 << 3; /* undefined */
If left shift is performed on a positive value and result of the mathematical value is not representable in the type, it's undefined1:
/* Assuming an int is 32-bits wide, the value '5 * 2^72' doesn't fit * in an int. So, this is undefined. */ int x = 5 << 72;
Note that right shift on a negative value (.e.g
-5 >> 3) is not undefined but implementation-defined.
1 Quoting ISO/IEC 9899:201x, section 6.5.7:
If the value of the right operand is negative or is greater than or equal to the width of the promoted left operand, the behavior is undefined.