fix(gnovm): handle blank range key/value per-operand, validate assignment targets

gnovm/pkg/gnolang/type_check.go

@@ -820,6 +820,9 @@ func (x *IncDecStmt) AssertCompatible(t Type) {
 }
 
 func assertIndexTypeIsInt(kt Type) {
+	if kt == nil {
+		return
+	}
 	if kt.Kind() != IntKind {
 		panic(fmt.Sprintf("index type should be int, but got %v", kt))
 	}
@@ -861,9 +864,7 @@ func (x *RangeStmt) AssertCompatible(store Store, last BlockNode) {
 		}
 	case PrimitiveType:
 		if cxt.Kind() == StringKind {
-			if kt != nil && kt.Kind() != IntKind {
-				panic(fmt.Sprintf("index type should be int, but got %v", kt))
-			}
+			assertIndexTypeIsInt(kt)
 			if vt != nil {
 				if vt.Kind() != Int32Kind { // rune
 					panic(fmt.Sprintf("value type should be int32, but got %v", kt))

gnovm/tests/files/range_blank_key.gno

@@ -0,0 +1,11 @@
+package main
+
+// Regression: `for _ = range slice` used to panic on nil key in assertIndexTypeIsInt.
+
+func main() {
+	s := []int{1, 2, 3}
+	for _ = range s {
+	}
+}
+
+// Output: