checker: fix some specific interface generic inference within generics struct and method (#20932)

Author: GGRei

vlib/v/checker/interface.v

@@ -303,10 +303,16 @@ fn (mut c Checker) resolve_generic_interface(typ ast.Type, interface_type ast.Ty
 					}
 					for i, iparam in imethod.params {
 						param := method.params[i] or { ast.Param{} }
-						if iparam.typ.has_flag(.generic) {
+						mut need_inferred_type := false
+						if !iparam.typ.has_flag(.generic) && iparam.typ == param.typ
+							&& imethod.return_type == method.return_type
+							&& imethod.name == method.name {
+							need_inferred_type = true
+						}
+						if iparam.typ.has_flag(.generic) || need_inferred_type {
 							param_sym := c.table.sym(iparam.typ)
 							arg_sym := c.table.sym(param.typ)
-							if c.table.get_type_name(iparam.typ) == gt_name {
+							if c.table.get_type_name(iparam.typ) == gt_name || need_inferred_type {
 								inferred_type = param.typ
 							} else if arg_sym.info is ast.Array && param_sym.info is ast.Array {
 								mut arg_elem_typ, mut param_elem_typ := arg_sym.info.elem_type, param_sym.info.elem_type

vlib/v/tests/generics_interface_with_generic_method_using_generic_struct_test.v

@@ -0,0 +1,25 @@
+struct Store[Z] {
+mut:
+	event_handlers []IEventHandler[Z]
+}
+
+fn (mut me Store[Z]) add_event_handler(event_handler IEventHandler[Z]) {
+	me.event_handlers << IEventHandler[Z](event_handler)
+}
+
+interface IEventHandler[Z] {
+mut:
+	apply(event Z)
+}
+
+struct UserEvent {}
+
+struct UserEventHandler {}
+
+pub fn (mut me UserEventHandler) apply(event UserEvent) {}
+
+fn test_generic_interface_with_method_using_generic_struct() {
+	mut s := Store[UserEvent]{}
+	s.add_event_handler(UserEventHandler{})
+	assert s.event_handlers.len > 0
+}