cmd/compile: adjust types2 shift check to match go/types (cleanup)

With this change, the shift checking code matches the corresponding
go/types code, but for the differences in the internal error reporting,
and call of check.overflow.

The change leads to the recording of an untyped int value if the RHS
of a non-constant shift is an untyped integer value. Adjust the type
in the compiler's irgen accordingly. Add test/shift3.go to verify
behavior.

Change-Id: I20386fcb1d5c48becffdc2203081fb70c08b282d
Reviewed-on: https://go-review.googlesource.com/c/go/+/398236
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
Trust: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

src/cmd/compile/internal/noder/expr.go

@@ -6,6 +6,7 @@ package noder
 
 import (
 	"fmt"
+	"go/constant"
 
 	"cmd/compile/internal/base"
 	"cmd/compile/internal/ir"
@@ -62,6 +63,14 @@ func (g *irgen) expr(expr syntax.Expr) ir.Node {
 		case types2.UntypedNil:
 			// ok; can appear in type switch case clauses
 			// TODO(mdempsky): Handle as part of type switches instead?
+		case types2.UntypedInt, types2.UntypedFloat, types2.UntypedComplex:
+			// Untyped rhs of non-constant shift, e.g. x << 1.0.
+			// If we have a constant value, it must be an int >= 0.
+			if tv.Value != nil {
+				s := constant.ToInt(tv.Value)
+				assert(s.Kind() == constant.Int && constant.Sign(s) >= 0)
+			}
+			typ = types2.Typ[types2.Uint]
 		case types2.UntypedBool:
 			typ = types2.Typ[types2.Bool] // expression in "if" or "for" condition
 		case types2.UntypedString:

src/cmd/compile/internal/types2/api_test.go

@@ -311,6 +311,18 @@ 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.`, `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
 		{brokenPkg + `x0; func _() { var x struct {f string}; x.f := 0 }`, `x.f`, `string`},
 		{brokenPkg + `x1; func _() { var z string; type x struct {f string}; y := &x{q: z}}`, `z`, `string`},

src/cmd/compile/internal/types2/expr.go

@@ -954,32 +954,48 @@ func (check *Checker) shift(x, y *operand, e syntax.Expr, op syntax.Operator) {
 	// 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.errorf(y, invalidOp+"negative shift count %s", y)
 			x.mode = invalid
 			return
 		}
-	}
 
-	// Caution: Check for isUntyped first because isInteger includes untyped
+		if isUntyped(y.typ) {
-	//          integers (was bug #43697).
+			// Caution: Check for representability here, rather than in the switch
-	if isUntyped(y.typ) {
+			// below, because isInteger includes untyped integers (was bug #43697).
-		check.convertUntyped(y, Typ[Uint])
+			check.representable(y, Typ[Uint])
-		if y.mode == invalid {
+			if y.mode == invalid {
+				x.mode = invalid
+				return
+			}
+		}
+	} 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.errorf(y, invalidOp+"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.errorf(y, invalidOp+"shift count %s must be integer", y)
 			x.mode = invalid
 			return
 		}
-	} else if !allInteger(y.typ) {
-		check.errorf(y, invalidOp+"shift count %s must be integer", y)
-		x.mode = invalid
-		return
-	} else if !allUnsigned(y.typ) && !check.allowVersion(check.pkg, 1, 13) {
-		check.versionErrorf(y, "go1.13", invalidOp+"signed shift count %s", y)
-		x.mode = invalid
-		return
 	}
 
 	if x.mode == constant_ {

src/cmd/compile/internal/types2/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 p
+
+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" */)

test/shift3.go

@@ -0,0 +1,41 @@
+// run
+
+// 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.
+
+// Test that the compiler's noder uses the correct type
+// for RHS shift operands that are untyped. Must compile;
+// run for good measure.
+
+package main
+
+import (
+	"fmt"
+	"math"
+)
+
+func f(x, y int) {
+	if x != y {
+		panic(fmt.Sprintf("%d != %d", x, y))
+	}
+}
+
+func main() {
+	var x int = 1
+	f(x<<1, 2)
+	f(x<<1., 2)
+	f(x<<(1+0i), 2)
+	f(x<<0i, 1)
+
+	f(x<<(1<<x), 4)
+	f(x<<(1.<<x), 4)
+	f(x<<((1+0i)<<x), 4)
+	f(x<<(0i<<x), 1)
+
+	// corner cases
+	const M = math.MaxUint
+	f(x<<(M+0), 0)     // shift by untyped int representable as uint
+	f(x<<(M+0.), 0)    // shift by untyped float representable as uint
+	f(x<<(M+0.+0i), 0) // shift by untyped complex representable as uint
+}