And this:

package main

func main() {
	s := []byte{1 << 31: 1} // error: index must be non-negative integer constant
	_ = s[1 << 62] // compile ok
	_ = s[1 << 63] // error:  constant 9223372036854775808 overflows int
	// I think the rules for the indexes in literal and the indexes in [] should be consistent.
}

This happens because the smallinstconst function in const.go has the following code:

if !ok || vi.CmpInt64(0) < 0 || vi.Cmp(maxintval[TINT32]) > 0 {
	return -1
}

The key you used fails the last check (vi.Cmp(maxintval[TINT32]) > 0) because maxintval[TINT32] is 0x7fffffff which is smaller than 1<<31, so the function returns -1 to signal a key error.

The problem here is that AFAIK we can't just easily remove the check above, because there are several places in the compiler (both in the frontend and the backend) where those values are assumed to be int32s.

cc @griesemer @randall77

I guess we could change smallinstconst to better signal the kind of errors it encounters, and make the callers check the error before printing a must be positive error. We could add a second error that says "too big".

I'll send a patch for 1.11 if this sounds good. There's still an issue with the spec wording, but at least we'll have a better error message.

FWIW, both go/types and gccgo accept this code w/o problems. gccgo can also run it (again w/o problems).

Change https://golang.org/cl/151599 mentions this issue: cmd/compile: fix constant index bounds check and error message