cmd/compile/internal/types2: better error message for invalid range clause

Fixes #50372.

Change-Id: I8e4c0020dae42744cce016433e398e0b884bb044
Reviewed-on: https://go-review.googlesource.com/c/go/+/375475
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

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

@@ -810,32 +810,34 @@ func (check *Checker) typeSwitchStmt(inner stmtContext, s *syntax.SwitchStmt, gu
 func (check *Checker) rangeStmt(inner stmtContext, s *syntax.ForStmt, rclause *syntax.RangeClause) {
 	// scope already opened
 
-	// check expression to iterate over
-	var x operand
-	check.expr(&x, rclause.X)
-
 	// determine lhs, if any
 	sKey := rclause.Lhs // possibly nil
-	var sValue syntax.Expr
+	var sValue, sExtra syntax.Expr
 	if p, _ := sKey.(*syntax.ListExpr); p != nil {
-		if len(p.ElemList) != 2 {
+		if len(p.ElemList) < 2 {
 			check.error(s, invalidAST+"invalid lhs in range clause")
 			return
 		}
+		// len(p.ElemList) >= 2
 		sKey = p.ElemList[0]
 		sValue = p.ElemList[1]
+		if len(p.ElemList) > 2 {
+			// delay error reporting until we know more
+			sExtra = p.ElemList[2]
+		}
 	}
 
+	// check expression to iterate over
+	var x operand
+	check.expr(&x, rclause.X)
+
 	// determine key/value types
 	var key, val Type
 	if x.mode != invalid {
 		// Ranging over a type parameter is permitted if it has a structural type.
 		var cause string
 		u := structuralType(x.typ)
-		switch t := u.(type) {
-		case nil:
-			cause = check.sprintf("%s has no structural type", x.typ)
-		case *Chan:
+		if t, _ := u.(*Chan); t != nil {
 			if sValue != nil {
 				check.softErrorf(sValue, "range over %s permits only one iteration variable", &x)
 				// ok to continue
@@ -843,6 +845,14 @@ func (check *Checker) rangeStmt(inner stmtContext, s *syntax.ForStmt, rclause *s
 			if t.dir == SendOnly {
 				cause = "receive from send-only channel"
 			}
+		} else {
+			if sExtra != nil {
+				check.softErrorf(sExtra, "range clause permits at most two iteration variables")
+				// ok to continue
+			}
+			if u == nil {
+				cause = check.sprintf("%s has no structural type", x.typ)
+			}
 		}
 		key, val = rangeKeyVal(u)
 		if key == nil || cause != "" {

src/cmd/compile/internal/types2/testdata/fixedbugs/issue50372.go

@@ -0,0 +1,27 @@
+// Copyright 2021 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
+
+func _(s []int) {
+        var i, j, k, l int
+        _, _, _, _ = i, j, k, l
+
+        for range s {}
+        for i = range s {}
+        for i, j = range s {}
+        for i, j, k /* ERROR range clause permits at most two iteration variables */ = range s {}
+        for i, j, k /* ERROR range clause permits at most two iteration variables */, l = range s {}
+}
+
+func _(s chan int) {
+        var i, j, k, l int
+        _, _, _, _ = i, j, k, l
+
+        for range s {}
+        for i = range s {}
+        for i, j /* ERROR range over .* permits only one iteration variable */ = range s {}
+        for i, j /* ERROR range over .* permits only one iteration variable */, k = range s {}
+        for i, j /* ERROR range over .* permits only one iteration variable */, k, l = range s {}
+}

test/fixedbugs/issue50372.go

@@ -0,0 +1,29 @@
+// errorcheck -G=3
+
+// 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
+
+func _(s []int) {
+        var i, j, k, l int
+        _, _, _, _ = i, j, k, l
+
+        for range s {}
+        for i = range s {}
+        for i, j = range s {}
+        for i, j, k = range s {} // ERROR "range clause permits at most two iteration variables"
+        for i, j, k, l = range s {} // ERROR "range clause permits at most two iteration variables"
+}
+
+func _(s chan int) {
+        var i, j, k, l int
+        _, _, _, _ = i, j, k, l
+
+        for range s {}
+        for i = range s {}
+        for i, j = range s {} // ERROR "range over .* permits only one iteration variable"
+        for i, j, k = range s {} // ERROR "range over .* permits only one iteration variable"
+        for i, j, k, l = range s {} // ERROR "range over .* permits only one iteration variable"
+}