go.tools/go/types: constant shifts always produce integers (bug fix)

R=adonovan
CC=golang-dev
https://golang.org/cl/12025044

go/types/expr.go

@@ -546,7 +546,13 @@ func (check *checker) shift(x, y *operand, op token.Token) {
 				x.mode = invalid
 				return
 			}
-			// everything's ok
+			// The lhs is representable as an integer but may not be an integer
+			// (e.g., 2.0, an untyped float) - this can only happen for untyped
+			// non-integer numeric constants. Correct the type so that the shift
+			// result is of integer type.
+			if !isInteger(x.typ) {
+				x.typ = Typ[UntypedInt]
+			}
 			x.val = exact.Shift(x.val, op, uint(s))
 			return
 		}

go/types/stdlib_test.go

@@ -63,10 +63,6 @@ func TestStdtest(t *testing.T) {
 		case "map1.go":
 			// TODO(gri) fix map key checking
 			continue
-		case "shift2.go":
-			// TODO(gri) fix constant shift expression error
-			// (default type for 1.0<<1 is int, not float64).
-			continue
 		case "sizeof.go", "switch.go":
 			// TODO(gri) tone down duplicate checking in expression switches
 			continue

go/types/testdata/shifts.src

@@ -4,8 +4,31 @@
 
 package shifts
 
+func shifts0() {
+	// basic constant shifts
+	const (
+		s = 10
+		_ = 0<<0
+		_ = 1<<s
+		_ = 1<<- /* ERROR "stupid shift" */ 1
+		_ = 2.0<<1
+
+		_ int = 2<<s
+		_ float32 = 2<<s
+		_ complex64 = 2<<s
+
+		_ int = 2.0<<s
+		_ float32 = 2.0<<s
+		_ complex64 = 2.0<<s
+
+		_ int = 'a'<<s
+		_ float32 = 'a'<<s
+		_ complex64 = 'a'<<s
+	)
+}
+
 func shifts1() {
-	// basics
+	// basic non-constant shifts
 	var (
 		i0 int
 		u0 uint