Enable inheritance of CAN enum types

This commit changes the `interface::can` namespace to a `struct`. This allows the enum types to be inherited and prevents breaking old code relying on referencing eg: `CAN::RxIrq`.

When enabled, the polymorphic CAN interface class inherits from this `interface::can` struct. If not enabled, the `mbed::CAN` class inherits from `interface::can` directly.

Co-authored-by: Vincent Coubard <[email protected]>

Author: AGlass0fMilk

drivers/include/drivers/CAN.h

@@ -41,7 +41,7 @@ class CAN
 #ifdef FEATURE_EXPERIMENTAL_API
     final : public interface::CAN, private NonCopyable<CAN>
 #else
-    : private NonCopyable<CAN>
+    : private NonCopyable<CAN>, public interface::can
 #endif
 {
 
@@ -161,8 +161,6 @@ class CAN
      */
     void monitor(bool silent);
 
-    using Mode = ::mbed::interface::can::Mode;
-
     /** Change CAN operation to the specified mode
      *
      *  @param mode The new operation mode (CAN::Normal, CAN::Silent, CAN::LocalTest, CAN::GlobalTest, CAN::SilentTest)
@@ -198,8 +196,6 @@ class CAN
      */
     unsigned char tderror();
 
-    using IrqType = ::mbed::interface::can::IrqType;
-
     /** Attach a function to call whenever a CAN frame received interrupt is
      *  generated.
      *

drivers/include/drivers/interfaces/InterfaceCAN.h

@@ -113,7 +113,8 @@ class CANMessage : public CAN_Message {
 
 namespace interface {
 
-namespace can {
+/* Having this as a struct allows interface::CAN and/or mbed::CAN to inherit the enums */
+struct can {
 
 enum Mode {
     Reset = 0,
@@ -138,15 +139,19 @@ enum IrqType {
     IrqCnt
 };
 
-} // namespace can
+// Prevent slicing and user creation of base class.
+protected:
+    can() = default;
+    ~can() = default;
+    can(const can&) = default;
+    can& operator=(const can&) = default;
+
+};
 
 #ifdef FEATURE_EXPERIMENTAL_API
 
 // Pure virtual interface for CAN
-struct CAN {
-
-    using Mode = ::mbed::interface::can::Mode;
-    using IrqType = ::mbed::interface::can::IrqType;
+struct CAN : public interface::can {
 
     virtual ~CAN() = default;
     virtual int frequency(int hz) = 0;