callback - Relax type-deduction in callback class

[geky]

This is a fix for a set of bugs related to using inherited member functions.

---

The type deduction for the callback constructors was to strict and prevented implicit casts for the context pointer stored internally.

As noted by @pan-, relaxing the contraints on the templated parameters allows C++ to correctly infer implicit casts such as conversions between child and parent classes when inheritance is
involved.

As an additional benefit, this may help the user experience by defering invalid type errors to when the types are expanded, limiting the number of error messages presented to users.

Also, adopting the same relaxed tpye-deduction in bound functions for consistency provides an alternative solution for the void pointer cast issue, which removes a larg amount of cruft.

---

Github is breaking on the diff, so here is a snippet of what has changed:

--- a/platform/Callback.h
+++ b/platform/Callback.h
@@ -90,8 +90,8 @@ public:
      *  @param obj      Pointer to object to invoke member function on
      *  @param method   Member function to attach
      */
-    template<typename T>
-    Callback(T *obj, R (T::*method)()) {
+    template<typename T, typename U>
+    Callback(U *obj, R (T::*method)()) {
         generate(method_context<T, R (T::*)()>(obj, method));
     }

@@ -117,49 +117,17 @@ public:
     /** Create a Callback with a static function and bound pointer
      *  @param func     Static function to attach
      *  @param arg      Pointer argument to function
      */
-    Callback(R (*func)(void*), void *arg) {
-        generate(function_context<R (*)(void*), void>(func, arg));
-    }
-
-    /** Create a Callback with a static function and bound pointer
-     *  @param func     Static function to attach
-     *  @param arg      Pointer argument to function
-     */
-    template<typename T>
-    Callback(R (*func)(T*), T *arg) {
+    template<typename T, typename U>
+    Callback(R (*func)(T*), U *arg) {
         generate(function_context<R (*)(T*), T>(func, arg));
     }

related #3235
cc @pan-

[pan-]

LGTM

[geky]

/morph test

[sg-]

/morph test

[mbed-bot]

Result: FAILURE

Your command has finished executing! Here's what you wrote!

/morph test

Output

mbed Build Number: 1075

Build Prep failed!

[bridadan]

We are currently experiencing internal network issues, the build prep failed because the repo failed to clone within 10 minutes. I'd recommend holding off on CI for the time being until the network issues are resolved.

[bridadan]

/morph test-nightly

[mbed-bot]

Result: FAILURE

Your command has finished executing! Here's what you wrote!

/morph test-nightly

Output

mbed Build Number: 1077

Build failed!

[bridadan]

Not an actual failure, there was a disk issue that should be resolved now.

/morph test-nightly

[mbed-bot]

Result: SUCCESS

Your command has finished executing! Here's what you wrote!

/morph test-nightly

Output

mbed Build Number: 1078

All builds and test passed!