what's wrong with golang constant overflows uint64

Question

userid := 12345
did := (userid & ^(0xFFFF << 48))

when compiling this code, I got:

./xxxx.go:511: constant -

Answer 1

The Go Programming Language Specification

Constants

Numeric constants represent values of arbitrary precision and do not overflow.

Constants may be typed or untyped.

A constant may be given a type explicitly by a constant declaration or conversion, or implicitly when used in a variable declaration or an assignment or as an operand in an expression. It is an error if the constant value cannot be represented as a value of the respective type.

An untyped constant has a default type which is the type to which the constant is implicitly converted in contexts where a typed value is required, for instance, in a short variable declaration such as i := 0 where there is no explicit type. The default type of an untyped constant is bool, rune, int, float64, complex128 or string respectively, depending on whether it is a boolean, rune, integer, floating-point, complex, or string constant.

Numeric types

int is an implementation-specific size, either 32 or 64 bits.

userid is of type int. For example,

package main

import "fmt"

func main() {
    userid := 12345
    did := uint64(userid) & ^uint64(0xFFFF<<48)
    fmt.Println(userid, did)
}

Output:

12345 12345