Callback updates

* Optimise clearing by adding `nullptr` overload. This overload means
  `Callback(NULL)` or `Callback(0)` will no longer work; users must
  use `Callback(nullptr)` or `Callback()`.
* Optimise clearing by not clearing storage - increases code size of
  comparison, but that is extremely rare.
* Reduce ROM used by trivial functors - share copy/destroy code.
* Config option to force trivial functors - major ROM saving by
  eliminating the "operations" table.
* Config option to eliminate comparison altogether - minor ROM saving by
  eliminating zero padding.
* Conform more to `std::function` API.

Author: kjbracey

TESTS/mbed_platform/Transaction/main.cpp

@@ -58,7 +58,11 @@ void test_Transaction_init()
     TEST_ASSERT_EQUAL(rx_buffer_size,   test_transaction.get_transaction()->rx_length);
     TEST_ASSERT_EQUAL(event_id,         test_transaction.get_transaction()->event);
     TEST_ASSERT_EQUAL(word_width,       test_transaction.get_transaction()->width);
-    TEST_ASSERT_EQUAL(callback,         test_transaction.get_transaction()->callback);
+#if MBED_CONF_PLATFORM_CALLBACK_COMPARABLE
+    TEST_ASSERT_TRUE(callback == test_transaction.get_transaction()->callback);
+#else
+    TEST_ASSERT_FALSE(nullptr == test_transaction.get_transaction()->callback)
+#endif
 }
 
 /** Test Transaction class - creation without initialisation

TESTS/network/emac/emac_util.h

@@ -102,8 +102,6 @@ char emac_if_get_trace_level();
 
 void emac_if_trace_to_ascii_hex_dump(const char *prefix, int len, char *data);
 
-void emac_if_link_state_change_cb(void *data, bool up);
-
 unsigned char *emac_if_get_own_addr(void);
 
 int emac_if_get_mtu_size();

UNITTESTS/features/netsocket/NetworkInterface/unittest.cmake

@@ -3,6 +3,8 @@
 # UNIT TESTS
 ####################
 
+set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -DMBED_CONF_PLATFORM_CALLBACK_COMPARABLE")
+
 # Source files
 set(unittest-sources
   ../features/netsocket/SocketAddress.cpp

UNITTESTS/target_h/platform/cxxsupport/mstd_new

@@ -0,0 +1,56 @@
+/* mbed Microcontroller Library
+ * Copyright (c) 2019 ARM Limited
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+#ifndef MSTD_NEW_
+#define MSTD_NEW_
+
+/* <mstd_new>
+ *
+ * - includes toolchain's <new>
+ * - For all toolchains, C++17 backports:
+ *   - mstd::launder
+ */
+
+#include <new>
+#if __cpp_lib_launder < 201606
+#include <type_traits>
+#endif
+
+namespace mstd
+{
+using std::nothrow_t;
+using std::nothrow;
+using std::new_handler;
+using std::set_new_handler;
+
+#if __cpp_lib_launder >= 201606
+using std::launder;
+#else
+template <typename T>
+constexpr T *launder(T *p) noexcept
+{
+    static_assert(!std::is_function<T>::value && !std::is_void<T>::value, "Can only launder complete object types");
+#if defined __clang__ || __GNUC__ >= 9
+    return __builtin_launder(p);
+#else
+    return p;
+#endif
+}
+#endif
+
+} // namespace mstd
+
+#endif // MSTD_NEW_

UNITTESTS/target_h/platform/cxxsupport/mstd_type_traits

@@ -300,20 +300,6 @@ struct remove_cvref : type_identity<std::remove_cv_t<std::remove_reference_t<T>>
 template <typename T>
 using remove_cvref_t = typename remove_cvref<T>::type;
 
-/* C++20 unwrap_reference */
-template <typename T>
-struct unwrap_reference : type_identity<T> { };
-template <typename T>
-struct unwrap_reference<std::reference_wrapper<T>> : type_identity<T &> { };
-template <typename T>
-using unwrap_reference_t = typename unwrap_reference<T>::type;
-
-/* C++20 unwrap_ref_decay */
-template <typename T>
-struct unwrap_ref_decay : unwrap_reference<std::decay_t<T>> { };
-template <typename T>
-using unwrap_ref_decay_t = typename unwrap_ref_decay<T>::type;
-
 }
 
 #if __cpp_lib_invoke < 201411

features/netsocket/NetworkInterface.cpp

@@ -141,6 +141,7 @@ void NetworkInterface::add_event_listener(mbed::Callback<void(nsapi_event_t, int
     attach(mbed::callback(&call_all_event_listeners, this));
 }
 
+#if MBED_CONF_PLATFORM_CALLBACK_COMPARABLE
 void NetworkInterface::remove_event_listener(mbed::Callback<void(nsapi_event_t, intptr_t)> status_cb)
 {
     iface_eventlist_t *event_list = get_interface_event_list_head();
@@ -152,6 +153,7 @@ void NetworkInterface::remove_event_listener(mbed::Callback<void(nsapi_event_t,
         }
     }
 }
+#endif
 
 NetworkInterface::~NetworkInterface()
 {

features/netsocket/NetworkInterface.h

@@ -352,13 +352,15 @@ class NetworkInterface: public DNS {
      */
     void add_event_listener(mbed::Callback<void(nsapi_event_t, intptr_t)> status_cb);
 
+#if MBED_CONF_PLATFORM_CALLBACK_COMPARABLE
     /** Remove event listener from interface.
      *
      * Remove previously added callback from the handler list.
      *
      *  @param status_cb The callback to unregister.
      */
     void remove_event_listener(mbed::Callback<void(nsapi_event_t, intptr_t)> status_cb);
+#endif
 
     /** Get the connection status.
      *