ast: fix generics with nested generic type parameter (fix #13077) (#13088)

vlib/v/ast/table.v

@@ -1540,6 +1540,7 @@ pub fn (mut t Table) resolve_generic_to_concrete(generic_type Type, generic_name
 pub fn (mut t Table) unwrap_generic_type(typ Type, generic_names []string, concrete_types []Type) Type {
 	mut final_concrete_types := []Type{}
 	mut fields := []StructField{}
+	mut needs_unwrap_types := []Type{}
 	mut nrt := ''
 	mut c_nrt := ''
 	ts := t.sym(typ)
@@ -1627,6 +1628,20 @@ pub fn (mut t Table) unwrap_generic_type(typ Type, generic_names []string, concr
 				}
 				if final_concrete_types.len > 0 {
 					for method in ts.methods {
+						for i in 1 .. method.params.len {
+							if method.params[i].typ.has_flag(.generic)
+								&& method.params[i].typ != method.params[0].typ {
+								if method.params[i].typ !in needs_unwrap_types {
+									needs_unwrap_types << method.params[i].typ
+								}
+							}
+							if method.return_type.has_flag(.generic)
+								&& method.return_type != method.params[0].typ {
+								if method.return_type !in needs_unwrap_types {
+									needs_unwrap_types << method.return_type
+								}
+							}
+						}
 						t.register_fn_concrete_types(method.fkey(), final_concrete_types)
 					}
 				}
@@ -1648,6 +1663,9 @@ pub fn (mut t Table) unwrap_generic_type(typ Type, generic_names []string, concr
 				mod: ts.mod
 				info: info
 			)
+			for typ_ in needs_unwrap_types {
+				t.unwrap_generic_type(typ_, generic_names, concrete_types)
+			}
 			return new_type(new_idx).derive(typ).clear_flag(.generic)
 		}
 		Interface {

vlib/v/tests/generics_with_nested_generic_type_parameter_test.v

@@ -0,0 +1,20 @@
+module main
+
+pub struct Randomizer<T> {
+}
+
+pub struct Element<T> {
+}
+
+fn test_generics_with_nested_generic_type_parameter() {
+	a := new_randomizer<int>()
+	println(a)
+	assert '$a' == '&Randomizer<int>{}'
+}
+
+pub fn new_randomizer<T>() &Randomizer<T> {
+	return &Randomizer<T>{}
+}
+
+pub fn (mut r Randomizer<T>) add_multiple(elements []Element<T>) {
+}