cmd/compile: fix typecheck range over negative integer

Before range over integer, types2 leaves constant expression in RHS of
non-constant shift untyped, so idealType do the validation to ensure
that constant value must be an int >= 0.

With range over int, the range expression can also be left untyped, and
can be an negative integer, causing the validation false.

Fixing this by relaxing the validation in idealType, and moving the
check to Unified IR reader.

Fixes #63378

Change-Id: I43042536c09afd98d52c5981adff5dbc5e7d882a
Reviewed-on: https://go-review.googlesource.com/c/go/+/532835
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -87,13 +87,14 @@ func idealType(tv syntax.TypeAndValue) types2.Type {
 			// 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.
+			typ = types2.Typ[types2.Uint]
 			if tv.Value != nil {
 				s := constant.ToInt(tv.Value)
-				assert(s.Kind() == constant.Int && constant.Sign(s) >= 0)
+				assert(s.Kind() == constant.Int)
+				if constant.Sign(s) < 0 {
+					typ = types2.Typ[types2.Int]
+				}
 			}
-			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/noder/reader.go

@@ -2233,6 +2233,13 @@ func (r *reader) expr() (res ir.Node) {
 		switch op {
 		case ir.OANDAND, ir.OOROR:
 			return typecheck.Expr(ir.NewLogicalExpr(pos, op, x, y))
+		case ir.OLSH, ir.ORSH:
+			// Untyped rhs of non-constant shift, e.g. x << 1.0.
+			// If we have a constant value, it must be an int >= 0.
+			if ir.IsConstNode(y) {
+				val := constant.ToInt(y.Val())
+				assert(val.Kind() == constant.Int && constant.Sign(val) >= 0)
+			}
 		}
 		return typecheck.Expr(ir.NewBinaryExpr(pos, op, x, y))
 

test/range3.go

@@ -68,6 +68,14 @@ func testint3() {
 	}
 }
 
+// Issue #63378.
+func testint4() {
+	for i := range -1 {
+		_ = i
+		panic("must not be executed")
+	}
+}
+
 // test range over functions
 
 var gj int
@@ -377,6 +385,7 @@ func main() {
 	testint1()
 	testint2()
 	testint3()
+	testint4()
 	testfunc0()
 	testfunc1()
 	testfunc2()