Why Does Go Throw an Overflow Error When Using Untyped Constants in Bitwise Operations?

Understanding Integer Overflow in Go Constants

In Go programming, attempting to fit an excessively large value into an integer constant can lead to a compilation error. Consider the following code snippet:

<code class="go">userid := 12345
did := (userid & ^(0xFFFF << 48))</code>

Upon compiling this code, you may encounter the following error:

./xxxx.go:511: constant -18446462598732840961 overflows int

This error stems from the untyped nature of constant expressions in Go.

Untyped Constants and Integer Overflow

In the given code, the constant ^(0xFFFF << 48) has an untyped value, meaning it can hold an arbitrarily large integer. However, when attempting to perform a bitwise AND operation with the typed integer userid, the compiler attempts to cast the untyped constant to an int type. The resulting value, -18446462598732840961, exceeds the maximum representable value for an int, leading to the overflow error.

Resolving the Overflow Issue

To avoid this overflow error, it's essential to explicitly type the untyped constant. In this case, you can use the following code instead:

<code class="go">userid := 12345
did := (userid & (uint64(1<<48) - 1))</code>

By explicitly casting the constant to uint64 using uint64(1<<48) - 1, you ensure that both operands in the bitwise AND operation have the same type, eliminating the overflow issue.

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