go/types: preserve untyped constants on the RHS of a shift expression

CL 291316 fixed go/types to verify that untyped shift counts are
representable by uint, but as a side effect also converted their types
to uint.

Rearrange the logic to keep the check for representability, but not
actually convert untyped integer constants. Untyped non-integer
constants are still converted, to preserve the behavior of 1.16. This
behavior for non-integer types is a bug, filed as #47410.

Updates #47410
Fixes #47243

Change-Id: I5eab4aab35b97f932fccdee2d4a18623ee2ccad5
Reviewed-on: https://go-review.googlesource.com/c/go/+/337529
Trust: Robert Findley <rfindley@google.com>
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

3 files changed, 44 insertions, 9 deletions

diff --git a/src/go/types/api_test.go b/src/go/types/api_test.go
index f37b91d5a4..f964c656f9 100644
--- a/src/go/types/api_test.go
+++ b/src/go/types/api_test.go
@@ -322,6 +322,18 @@ func TestTypesInfo(t *testing.T) {
 			`[][]struct{}`,
 		},
 
+		// 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_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`},
+		{`package issue47243_f; var x int32; var _ = 1 << 2`, `2`, `untyped int`},
+		{`package issue47243_g; var x int32; var _ = int(1) << 2`, `2`, `untyped int`},
+		{`package issue47243_h; var x int32; var _ = 1 << (2 << x)`, `1`, `int`},
+		{`package issue47243_i; var x int32; var _ = 1 << (2 << x)`, `(2 << x)`, `untyped int`},
+		{`package issue47243_j; var x int32; var _ = 1 << (2 << x)`, `2`, `untyped int`},
+
 		// tests for broken code that doesn't parse or type-check
 		{broken + `x0; func _() { var x struct {f string}; x.f := 0 }`, `x.f`, `string`},
 		{broken + `x1; func _() { var z string; type x struct {f string}; y := &x{q: z}}`, `z`, `string`},
diff --git a/src/go/types/check_test.go b/src/go/types/check_test.go
index c85a8e46fb..f83abf11ce 100644
--- a/src/go/types/check_test.go
+++ b/src/go/types/check_test.go
@@ -344,6 +344,13 @@ func TestIssue46453(t *testing.T) {
 	checkFiles(t, nil, "", []string{"issue46453.go"}, [][]byte{[]byte(src)}, false, nil)
 }
 
+func TestIssue47243_TypedRHS(t *testing.T) {
+	// The RHS of the shift expression below overflows uint on 32bit platforms,
+	// but this is OK as it is explicitly typed.
+	const src = "package issue47243\n\nvar a uint64; var _ = a << uint64(4294967296)" // uint64(1<<32)
+	checkFiles(t, &StdSizes{4, 4}, "", []string{"p.go"}, [][]byte{[]byte(src)}, false, nil)
+}
+
 func TestCheck(t *testing.T)     { DefPredeclaredTestFuncs(); testDir(t, "check") }
 func TestExamples(t *testing.T)  { testDir(t, "examples") }
 func TestFixedbugs(t *testing.T) { testDir(t, "fixedbugs") }
diff --git a/src/go/types/expr.go b/src/go/types/expr.go
index 5c65fad447..58962e777b 100644
--- a/src/go/types/expr.go
+++ b/src/go/types/expr.go
@@ -778,32 +778,48 @@ func (check *Checker) shift(x, y *operand, e ast.Expr, op token.Token) {
 	// spec: "The right operand in a shift expression must have integer type
 	// or be an untyped constant representable by a value of type uint."
 
-	// Provide a good error message for negative shift counts.
+	// Check that constants are representable by uint, but do not convert them
+	// (see also issue #47243).
 	if y.mode == constant_ {
+		// Provide a good error message for negative shift counts.
 		yval := constant.ToInt(y.val) // consider -1, 1.0, but not -1.1
 		if yval.Kind() == constant.Int && constant.Sign(yval) < 0 {
 			check.invalidOp(y, _InvalidShiftCount, "negative shift count %s", y)
 			x.mode = invalid
 			return
 		}
+
+		if isUntyped(y.typ) {
+			// Caution: Check for representability here, rather than in the switch
+			// below, because isInteger includes untyped integers (was bug #43697).
+			check.representable(y, Typ[Uint])
+			if y.mode == invalid {
+				x.mode = invalid
+				return
+			}
+		}
 	}
 
-	// Caution: Check for isUntyped first because isInteger includes untyped
-	//          integers (was bug #43697).
-	if isUntyped(y.typ) {
+	// Check that RHS is otherwise at least of integer type.
+	switch {
+	case isInteger(y.typ):
+		if !isUnsigned(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
 		}
-	} else if !isInteger(y.typ) {
+	default:
 		check.invalidOp(y, _InvalidShiftCount, "shift count %s must be integer", y)
 		x.mode = invalid
 		return
-	} else if !isUnsigned(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
 	}
 
 	if x.mode == constant_ {