go/types: add missing test for constant shifts

Port of https://go-review.googlesource.com/11344 to std repo.

Fixes #11325.

Change-Id: I634beaf77cbaeb09de50aa1410e8c53fc37b19df
Reviewed-on: https://go-review.googlesource.com/11317Reviewed-by: Alan Donovan <adonovan@google.com>

src/go/types/expr.go

@@ -619,7 +619,7 @@ func (check *Checker) shift(x, y *operand, op token.Token) {
 
 	// The lhs must be of integer type or be representable
 	// as an integer; otherwise the shift has no chance.
-	if !isInteger(x.typ) && (!untypedx || !representableConst(x.val, nil, UntypedInt, nil)) {
+	if !x.isInteger() {
 		check.invalidOp(x.pos(), "shifted operand %s must be integer", x)
 		x.mode = invalid
 		return
@@ -645,6 +645,12 @@ func (check *Checker) shift(x, y *operand, op token.Token) {
 
 	if x.mode == constant {
 		if y.mode == constant {
+			// rhs must be an integer value
+			if !y.isInteger() {
+				check.invalidOp(y.pos(), "shift count %s must be unsigned integer", y)
+				x.mode = invalid
+				return
+			}
 			// rhs must be within reasonable bounds
 			const stupidShift = 1023 - 1 + 52 // so we can express smallestFloat64
 			s, ok := exact.Uint64Val(y.val)

src/go/types/testdata/shifts.src

@@ -319,3 +319,15 @@ func issue5895() {
 	var x = 'a' << 1 // type of x must be rune
 	var _ rune = x
 }
+
+func issue11325() {
+	var _ = 0 >> 1.1 /* ERROR "must be unsigned integer" */ // example from issue 11325
+	_ = 0 >> 1.1 /* ERROR "must be unsigned integer" */
+	_ = 0 << 1.1 /* ERROR "must be unsigned integer" */
+	_ = 0 >> 1.
+	_ = 1 >> 1.1 /* ERROR "must be unsigned integer" */
+	_ = 1 >> 1.
+	_ = 1. >> 1
+	_ = 1. >> 1.
+	_ = 1.1 /* ERROR "must be integer" */ >> 1
+}