For the built-in shift operator, the right operand must be nonnegative and strictly less than the bit width of the promoted left operand. Otherwise, the behavior is undefined.
const int a = 42;
const int b = a << -1; // UB
const int c = a << 0; // ok
const int d = a << 32; // UB if int is 32 bits or less
const int e = a >> 32; // also UB if int is 32 bits or less
const signed char f = 'x';
const int g = f << 10; // ok even if signed char is 10 bits or less;
// int must be at least 16 bits
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