Per the spec, the following code

var a []int
var s uint
var _ = a[0xffffffffffffffff << s]

is not permitted:

• the type of an untyped LHS in a non-constant shift is the type it would have without the shift
• the type of an untyped constant index must be non-negative and representable by a value of type int

Here, 0xffffffffffffffff does not fit into an int, but go/types accepts it.

cmd/compile reports: constant 18446744073709551615 overflows int