checker: prevent from casting non-struct to struct (#6143)

vlib/v/checker/checker.v

@@ -2333,12 +2333,18 @@ pub fn (mut c Checker) expr(node ast.Expr) table.Type {
 				type_name := c.table.type_to_str(node.expr_type)
 				c.error('cannot cast type `$type_name` to `byte`', node.pos)
 			} else if to_type_sym.kind == .struct_ && !node.typ.is_ptr() && !(to_type_sym.info as table.Struct).is_typedef {
+				// For now we ignore C typedef because of `C.Window(C.None)` in vlib/clipboard
+				if from_type_sym.kind == .struct_ && !node.expr_type.is_ptr() {
 					from_type_info := from_type_sym.info as table.Struct
 					to_type_info := to_type_sym.info as table.Struct
 					if !c.check_struct_signature(from_type_info, to_type_info) {
 						c.error('cannot convert struct `$from_type_sym.name` to struct `$to_type_sym.name`',
 							node.pos)
 					}
+				} else {
+					type_name := c.table.type_to_str(node.expr_type)
+					c.error('cannot cast `$type_name` to struct', node.pos)
+				}
 			}
 			if node.has_arg {
 				c.expr(node.arg)

vlib/v/checker/tests/cannot_cast_to_struct.out

@@ -0,0 +1,27 @@
+vlib/v/checker/tests/cannot_cast_to_struct.v:10:12: error: cannot convert struct `Abc` to struct `Test`
+    8 |
+    9 | fn main() {
+   10 |     _ := Test(Abc{})
+      |               ~~~~~
+   11 |     sum := Alphabet(Xyz{})
+   12 |     _ = Xyz(sum)
+vlib/v/checker/tests/cannot_cast_to_struct.v:12:10: error: cannot cast `Alphabet` to struct
+   10 |     _ := Test(Abc{})
+   11 |     sum := Alphabet(Xyz{})
+   12 |     _ = Xyz(sum)
+      |             ~~~
+   13 |     _ = Xyz(5)
+   14 |     s := Abc{}
+vlib/v/checker/tests/cannot_cast_to_struct.v:13:10: error: cannot cast `any_int` to struct
+   11 |     sum := Alphabet(Xyz{})
+   12 |     _ = Xyz(sum)
+   13 |     _ = Xyz(5)
+      |             ^
+   14 |     s := Abc{}
+   15 |     _ = Xyz(&s)
+vlib/v/checker/tests/cannot_cast_to_struct.v:15:10: error: cannot cast `&Abc` to struct
+   13 |     _ = Xyz(5)
+   14 |     s := Abc{}
+   15 |     _ = Xyz(&s)
+      |             ^
+   16 | }

vlib/v/checker/tests/cannot_cast_to_struct.vv

@@ -0,0 +1,16 @@
+struct Abc {}
+struct Xyz {}
+type Alphabet = Abc | Xyz
+
+struct Test {
+	abc Alphabet
+}
+
+fn main() {
+	_ := Test(Abc{})
+	sum := Alphabet(Xyz{})
+	_ = Xyz(sum)
+	_ = Xyz(5)
+	s := Abc{}
+	_ = Xyz(&s)
+}