Question

Normally, C requires that a binary operator’s operands are promoted to the type of the higher-ranking operand. This can be exploited to avoid filling code with verbose casts, for example:

if (x-48U<10) ...
y = x+0ULL << 40;

etc.

However, I’ve found that, at least with gcc, this behavior does not work for bitshifts. I.e.

int x = 1;
unsigned long long y = x << 32ULL;

I would expect the type of the right-hand operand to cause the left-hand operand to be promoted to unsigned long long so that the shift succeeds. But instead, gcc prints a warning:

warning: left shift count >= width of type

Is gcc broken, or does the standard make some exception to the type promotion rules for bitshifts?

Answer 1

The so-called usual arithmetic conversions apply to many binary operators, but not all of them. For example they do not apply to the bit shift operators, &&, ||, comma operator, and assignment operators. This is the rule for the bit shift operators:

6.5.7 … 3 Semantics …
The integer promotions are performed on each of the operands. The type of the result is that of the promoted left operand. 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.