cgen: fix generic return for option ptr (#17987)

Author: felipensp

vlib/v/gen/c/cgen.v

@@ -4755,7 +4755,7 @@ fn (mut g Gen) return_stmt(node ast.Return) {
 			styp := g.base_type(fn_ret_type)
 			g.writeln('${ret_typ} ${tmpvar};')
 			g.write('_option_ok(&(${styp}[]) { ')
-			if !fn_ret_type.is_ptr() && node.types[0].is_ptr() {
+			if !g.unwrap_generic(fn_ret_type).is_ptr() && node.types[0].is_ptr() {
 				if !(node.exprs[0] is ast.Ident && !g.is_amp) {
 					g.write('*')
 				}

vlib/v/tests/option_generic_ptr_return_test.v

@@ -0,0 +1,22 @@
+struct ObjectDesc {
+	typ u32
+	ptr voidptr
+}
+
+struct ABCD {}
+
+pub fn cast_object_desc[H](desc &ObjectDesc) ?H {
+	$if H is &ABCD {
+		if desc.typ == 12 { // desc == ABCD
+			return &ABCD(desc.ptr)
+		}
+	}
+	return none
+}
+
+fn test_main() {
+	obj := ABCD{}
+	a := cast_object_desc[&ABCD](ObjectDesc{ typ: 12, ptr: voidptr(&obj) })
+	dump(a)
+	assert *a? == obj
+}