cgen: fix code generated to option fn (fix #17604) (#17614)

vlib/v/gen/c/fn.v

@@ -571,7 +571,7 @@ fn (mut g Gen) fn_decl_params(params []ast.Param, scope &ast.Scope, is_variadic
 		}
 		param_type_sym := g.table.sym(typ)
 		mut param_type_name := g.typ(typ) // util.no_dots(param_type_sym.name)
-		if param_type_sym.kind == .function {
+		if param_type_sym.kind == .function && !typ.has_flag(.option) {
 			info := param_type_sym.info as ast.FnType
 			func := info.func
 			g.write('${g.typ(func.return_type)} (*${caname})(')

vlib/v/tests/option_fn_test.v

@@ -0,0 +1,47 @@
+type HandlerFn = fn ()
+
+fn new_static_router(route_not_found_handler ?fn ()) fn () {
+	return route_not_found_handler or {
+		fn () {}
+	}
+}
+
+fn new_static_router2(route_not_found_handler ?HandlerFn) fn () {
+	return route_not_found_handler or {
+		fn () {}
+	}
+}
+
+fn test_option_fn_alias_decl_with_none() {
+	b := new_static_router2(none)
+	$if b is $Function {
+		assert true
+	} $else {
+		assert false
+	}
+}
+
+fn test_option_fn_decl_with_none() {
+	a := new_static_router(none)
+	$if a is $Function {
+		assert true
+	} $else {
+		assert false
+	}
+}
+
+fn test_option_fn_passing_normal() {
+	anon_1 := fn () {
+		println(1)
+	}
+	c := new_static_router(anon_1)
+	assert c == anon_1
+}
+
+fn test_option_fn_passing_to_alias() {
+	anon_2 := fn () {
+		println(2)
+	}
+	d := new_static_router(anon_2)
+	assert d == anon_2
+}