checker: fix cast from an empty struct to option (fix #25566) (#25581)

Author: StunxFS

vlib/v/checker/checker.v

@@ -3550,7 +3550,8 @@ fn (mut c Checker) cast_expr(mut node ast.CastExpr) ast.Type {
 				c.warn('casting to struct is deprecated, use e.g. `Struct{...expr}` instead',
 					node.pos)
 			}
-			if !c.check_struct_signature(from_sym.info, to_sym.info) {
+			if from_type.idx() != to_type.idx()
+				&& !c.check_struct_signature(from_sym.info, to_sym.info) {
 				c.error('cannot convert struct `${from_sym.name}` to struct `${to_sym.name}`',
 					node.pos)
 			}

vlib/v/tests/casts/cast_empty_struct_to_option.v

@@ -0,0 +1,7 @@
+struct Struct {}
+
+fn main() {
+	s := ?Struct(Struct{})
+	a := s or { panic('none') }
+	assert a == Struct{}
+}