go/types, types2: constant with inferred type constrained by constraints.Integer not allowed as shifted operand

[fzipp]

What version of Go are you using (go version)?

$ go version
go version devel go1.18-91abe4be0e Sat Nov 20 08:47:36 2021 +0000 darwin/arm64

What did you do?

package main

import "constraints"

func shl[I constraints.Integer](n I) I {
	return 1 << n
}

func main() {}

What did you expect to see?

Successful compilation, because the non-generic function would compile for all types covered by constraints.Integer.

What did you see instead?

./prog.go:6:9: invalid operation: shifted operand 1 (type I) must be integer

The generic function compiles with an explicit type conversion:

func shl[I constraints.Integer](n I) I {
	return I(1) << n
}

[gopherbot]

Change https://golang.org/cl/365895 mentions this issue: cmd/compile/internal/types2: use allInteger to check type for shifted operand