checker: disallow assigning anon struct to typed struct (#18017)

vlib/v/checker/struct.v

@@ -574,6 +574,14 @@ fn (mut c Checker) struct_init(mut node ast.StructInit, is_field_zero_struct_ini
 						}
 					}
 				}
+
+				if field_type_sym.kind == .struct_ && !(field_type_sym.info as ast.Struct).is_anon
+					&& mut field.expr is ast.StructInit {
+					if field.expr.is_anon {
+						c.error('cannot assign anonymous `struct` to a typed `struct`',
+							field.expr.pos)
+					}
+				}
 			}
 			// Check uninitialized refs/sum types
 			// The variable `fields` contains two parts, the first part is the same as info.fields,

vlib/v/checker/tests/assign_anon_struct_to_typed_struct_err.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/assign_anon_struct_to_typed_struct_err.vv:11:18: error: cannot assign anonymous `struct` to a typed `struct`
+    9 |
+   10 | f := Fax{
+   11 |     message: struct {
+      |                     ^
+   12 |         a: 'A'
+   13 |     }

vlib/v/checker/tests/assign_anon_struct_to_typed_struct_err.vv

@@ -0,0 +1,16 @@
+struct Message {
+	a string
+	b string
+}
+
+struct Fax {
+	message Message
+}
+
+f := Fax{
+	message: struct {
+		a: 'A'
+	}
+}
+
+dump(f)