fix: correct typed-nil handling

gnovm/pkg/gnolang/preprocess.go

@@ -1231,9 +1231,9 @@
 						panic("type conversion requires single argument")
 					}
 					n.NumArgs = 1
+					ct := evalStaticType(store, last, n.Func)
 					if arg0, ok := n.Args[0].(*ConstExpr); ok {
 						var constConverted bool
-						ct := evalStaticType(store, last, n.Func)
 						// As a special case, if a decimal cannot
 						// be represented as an integer, it cannot be converted to one,
 						// and the error is handled here.
@@ -1263,6 +1263,19 @@
 							convertConst(store, last, arg0, nil)
 						}
 
+						// check legal type for nil
+						if arg0.IsUndefined() {
+							switch ct.Kind() { // special case for nil conversion check.
+							case SliceKind, PointerKind, FuncKind, MapKind, InterfaceKind:
+								//dt = ct // convert nil to typed-nil
+								convertConst(store, last, arg0, ct)
+							default:
+								panic(fmt.Sprintf(
+									"cannot convert %v to %v",
+									arg0, ct.Kind()))
+							}
+						}
+
 						// evaluate the new expression.
 						cx := evalConst(store, last, n)
 						// Though cx may be undefined if ct is interface,

gnovm/pkg/gnolang/values_conversions.go

@@ -73,6 +73,11 @@ func ConvertTo(alloc *Allocator, store Store, tv *TypedValue, t Type) {
 GNO_CASE:
 	// special case for interface target
 	if t.Kind() == InterfaceKind {
+		if tv.IsUndefined() && tv.T == nil {
+			if _, ok := t.(*NativeType); !ok { // no support for native now
+				tv.T = t
+			}
+		}
 		return
 	}
 	// special case for undefined/nil source

gnovm/tests/files/types/typed_nil_a.gno

@@ -0,0 +1,18 @@
+package main
+
+import "fmt"
+
+type integer int
+
+func main() {
+	// illegal conversion
+	// should not work
+	if integer(nil) == nil {
+		fmt.Println("integer is nil")
+	} else {
+		fmt.Println("integer is not nil")
+	}
+}
+
+// Error:
+// main/files/types/typed_nil_a.gno:10:5: cannot convert (undefined) to main.integer

gnovm/tests/files/types/typed_nil_b.gno

@@ -0,0 +1,16 @@
+package main
+
+import "fmt"
+
+type integer *int
+
+func main() {
+	println(integer(nil))
+	fmt.Println(integer(nil))
+	fmt.Printf("%T \n", integer(nil))
+}
+
+// Output:
+// (nil main.integer)
+// <nil>
+// *int

gnovm/tests/files/types/typed_nil_c.gno

@@ -0,0 +1,12 @@
+package main
+
+import "fmt"
+
+func main() {
+	println(interface{}(nil) == (*int)(nil))
+	fmt.Printf("%T \n", interface{}(nil))
+}
+
+// Output:
+// false
+// <nil>