Make it possible to call Encode/Decode on a subclass of a cluster-obj…

…ect struct. (#11014)

Our enable_if machinery for Encode/Decode only worked for structs of
type T if there was a T::Encode method.  That made it impossible to
subclass a struct (e.g. with the subclass providing backing storage
for the struct's Spans) and call Encode on the subclass instance
without jumping through some sort of testing hoops.

This change changes our test from "T::Encode exists with the right
signature" to "an instance of T has a .Encode that when called with
the right arguments returns the right return type", which lets
subclassing work as desired.

src/app/data-model/Decode.h

@@ -86,10 +86,11 @@ inline CHIP_ERROR Decode(TLV::TLVReader & reader, Span<const char> & x)
  * CHIP_ERROR <Object>::Decode(TLVReader &reader);
  *
  */
-template <
-    typename X,
-    typename std::enable_if_t<
-        std::is_class<X>::value && std::is_same<decltype(&X::Decode), CHIP_ERROR (X::*)(TLV::TLVReader &)>::value, X> * = nullptr>
+template <typename X,
+          typename std::enable_if_t<
+              std::is_class<X>::value &&
+                  std::is_same<decltype(std::declval<X>().Decode(std::declval<TLV::TLVReader &>())), CHIP_ERROR>::value,
+              X> * = nullptr>
 CHIP_ERROR Decode(TLV::TLVReader & reader, X & x)
 {
     return x.Decode(reader);

src/app/data-model/Encode.h

@@ -70,11 +70,12 @@ inline CHIP_ERROR Encode(TLV::TLVWriter & writer, TLV::Tag tag, Span<const char>
  *
  *
  */
-template <
-    typename X,
-    typename std::enable_if_t<std::is_class<X>::value &&
-                                  std::is_same<decltype(&X::Encode), CHIP_ERROR (X::*)(TLV::TLVWriter &, TLV::Tag) const>::value,
-                              X> * = nullptr>
+template <typename X,
+          typename std::enable_if_t<
+              std::is_class<X>::value &&
+                  std::is_same<decltype(std::declval<X>().Encode(std::declval<TLV::TLVWriter &>(), std::declval<TLV::Tag>())),
+                               CHIP_ERROR>::value,
+              X> * = nullptr>
 CHIP_ERROR Encode(TLV::TLVWriter & writer, TLV::Tag tag, const X & x)
 {
     return x.Encode(writer, tag);