checker: fix generic array.map with generic callback fn (#11991)

vlib/v/checker/checker.v

@@ -2683,7 +2683,7 @@ fn (mut c Checker) array_builtin_method_call(mut node ast.CallExpr, left_type as
 			ast.FnType { arg_sym.info.func.return_type }
 			else { arg_type }
 		}
-		node.return_type = c.table.find_or_register_array(ret_type)
+		node.return_type = c.table.find_or_register_array(c.unwrap_generic(ret_type))
 	} else if method_name == 'filter' {
 		// check fn
 		c.check_map_and_filter(false, elem_typ, node)

vlib/v/tests/generics_array_map_with_generic_callback_test.v

@@ -0,0 +1,21 @@
+fn test_generics_array_map_with_generic_callback() {
+	mut a := []Sth{}
+	a = arr_generic<Sth>(buggy_cb)
+	println(a)
+	assert a.len == 0
+}
+
+fn arr_generic<T>(cb fn (int) T) []T {
+	return []int{}.map(cb(it))
+}
+
+fn buggy_cb(a int) Sth {
+	return Sth{
+		a: a
+	}
+}
+
+struct Sth {
+	a int
+	b string
+}