}
			// x is a constant so xval != nil and it must be of Int kind.
			x.val = constant.Shift(xval, op, uint(s))
			x.expr = e
			opPos := x.Pos()
			if b, _ := e.(*ast.BinaryExpr); b != nil {
				opPos = b.OpPos
			}
			check.overflow(x, opPos)
			return
		}

		// non-constant shift with constant lhs
		if isUntyped(x.typ) {
			// spec: "If the left operand of a non-constant shift

			return false
		}
	} else {
		check.errorf(x, InvalidSyntaxTree, "unknown operator %s", op)
		return false
	}
	return true
}

// opPos returns the position of the operator if x is an operation;
// otherwise it returns the start position of x.
func opPos(x syntax.Expr) syntax.Pos {
	switch op := x.(type) {
	case nil:
		return nopos // don't crash
	case *syntax.Operation:
		return op.Pos()
	default: