[release-branch.go1.18] go/types: don't report errors for untyped int shifts on Go < 1.13

CL 337529 introduced upfront type-checking of constant shift operands,
to avoid converting their type to uint (per the spec). However, it
had an oversight in that the checks intended for non-constant operands
still ran after the explicit checking of constant operands. As a
result, there are at least two bugs:
 - When GoVersion is < 1.13, we report spurious errors for untyped
   constant shift operands.
 - When the operand is an untyped float constant, we still convert to
   uint (this was a known bug reported in #47410).

Looking at this now, it seems clear that we can avoid both of these bugs
by simply not running the additional checks in the case of a constant
operand. However, this should be considered with some care, as shifts
are notoriously tricky.

While cherry-picking, the new test file is updated to use the go1_12
package name, following our convention for specifying language version
in the release branch.

Fixes #52032

Change-Id: Ia489cc5470b92a8187d3de0423d05b309daf47bb
Reviewed-on: https://go-review.googlesource.com/c/go/+/396775
Reviewed-by: Robert Griesemer <gri@golang.org>
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
(cherry picked from commit 8a816d5efcc842ae92f0193aa9c1d433bd66ef31)
Reviewed-on: https://go-review.googlesource.com/c/go/+/397680

3 files changed, 53 insertions, 20 deletions

diff --git a/src/go/types/api_test.go b/src/go/types/api_test.go
index 6b05d66b10..78977ff2f0 100644
--- a/src/go/types/api_test.go
+++ b/src/go/types/api_test.go
@@ -330,7 +330,7 @@ func TestTypesInfo(t *testing.T) {
 
 		// issue 47243
 		{`package issue47243_a; var x int32; var _ = x << 3`, `3`, `untyped int`},
-		{`package issue47243_b; var x int32; var _ = x << 3.`, `3.`, `uint`}, // issue 47410: should be untyped float
+		{`package issue47243_b; var x int32; var _ = x << 3.`, `3.`, `untyped float`},
 		{`package issue47243_c; var x int32; var _ = 1 << x`, `1 << x`, `int`},
 		{`package issue47243_d; var x int32; var _ = 1 << x`, `1`, `int`},
 		{`package issue47243_e; var x int32; var _ = 1 << 2`, `1`, `untyped int`},
diff --git a/src/go/types/expr.go b/src/go/types/expr.go
index e24bd60dc3..23a735ce69 100644
--- a/src/go/types/expr.go
+++ b/src/go/types/expr.go
@@ -934,28 +934,28 @@ func (check *Checker) shift(x, y *operand, e ast.Expr, op token.Token) {
 				return
 			}
 		}
-	}
-
-	// Check that RHS is otherwise at least of integer type.
-	switch {
-	case allInteger(y.typ):
-		if !allUnsigned(y.typ) && !check.allowVersion(check.pkg, 1, 13) {
-			check.invalidOp(y, _InvalidShiftCount, "signed shift count %s requires go1.13 or later", y)
-			x.mode = invalid
-			return
-		}
-	case isUntyped(y.typ):
-		// This is incorrect, but preserves pre-existing behavior.
-		// See also bug #47410.
-		check.convertUntyped(y, Typ[Uint])
-		if y.mode == invalid {
+	} else {
+		// Check that RHS is otherwise at least of integer type.
+		switch {
+		case allInteger(y.typ):
+			if !allUnsigned(y.typ) && !check.allowVersion(check.pkg, 1, 13) {
+				check.invalidOp(y, _InvalidShiftCount, "signed shift count %s requires go1.13 or later", y)
+				x.mode = invalid
+				return
+			}
+		case isUntyped(y.typ):
+			// This is incorrect, but preserves pre-existing behavior.
+			// See also bug #47410.
+			check.convertUntyped(y, Typ[Uint])
+			if y.mode == invalid {
+				x.mode = invalid
+				return
+			}
+		default:
+			check.invalidOp(y, _InvalidShiftCount, "shift count %s must be integer", y)
 			x.mode = invalid
 			return
 		}
-	default:
-		check.invalidOp(y, _InvalidShiftCount, "shift count %s must be integer", y)
-		x.mode = invalid
-		return
 	}
 
 	if x.mode == constant_ {
diff --git a/src/go/types/testdata/fixedbugs/issue52031.go b/src/go/types/testdata/fixedbugs/issue52031.go
new file mode 100644
index 0000000000..dff2abd667
--- /dev/null
+++ b/src/go/types/testdata/fixedbugs/issue52031.go
@@ -0,0 +1,33 @@
+// -lang=go1.12
+
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package go1_12
+
+type resultFlags uint
+
+// Example from #52031.
+//
+// The following shifts should not produce errors on Go < 1.13, as their
+// untyped constant operands are representable by type uint.
+const (
+	_ resultFlags = (1 << iota) / 2
+
+	reportEqual
+	reportUnequal
+	reportByIgnore
+	reportByMethod
+	reportByFunc
+	reportByCycle
+)
+
+// Invalid cases.
+var x int = 1
+var _ = (8 << x /* ERROR "signed shift count .* requires go1.13 or later" */)
+
+const _ = (1 << 1.2 /* ERROR "truncated to uint" */)
+
+var y float64
+var _ = (1 << y /* ERROR "must be integer" */)