Enum types aren't validated

[EricRahm]

Background

Given the following definition:

enum McuCommand:
    ACK                  = 0x000000
    NACK                 = 0x000001
    BUSY                 = 0x000005

struct McuMessageHeader:
  0 [+1] McuCommand reason

I would expect the generated C++ code to validate the reason field to make sure it is a valid enum value. Or at least I'd want to have a helper that could validate it for me.

std::array<std::byte, 1> msg;
msg[0] = std::byte{static_cast<std::byte>(McuCommand::BUSY) + 1};
auto view = MakeMcuMessageHeaderView(&msg);
assert(!view.Ok()); // doesn't assert

Issue

We appear to use AllValuesAreOk as our validator for EnumView types, for example:

template <class Storage>
inline typename ::emboss::support::EnumView<
    /**/ ::McuCommand,
    ::emboss::support::FixedSizeViewParameters<24, ::emboss::support::AllValuesAreOk>,
    typename ::emboss::support::BitBlock</**/::emboss::support::LittleEndianByteOrderer<typename Storage::template OffsetStorageType</**/0, 6>>, 24>>

 GenericMcuMessageHeaderView<Storage>::reason()
    const;

This leads to, well, all values being ok.

Possible fixes

I see this as a few incremental improvements depending on what we want to do.

Generate a validator

Minmal of what we should do so that users can manually validate. Something like:

struct EnumTrait<::McuCommand> {
  static constexpr bool is_valid(std::underlying_type_t<::McuCommand> value) {
   switch(value) {
    case ::McuCommand::ACK:
    case ::McuCommand::NACK:
    case ::McuCommand::BUSY:
      return true;
    default:
      return false;
   }
  }
}

Add some min and max helpers

It might be useful for quick checks to include a min and max value. Something like:

struct EnumTrait<::McuCommand> {
  static constexpr std::underlying_type_t<::McuCommand> min = ::McuCommand::ACK;
  static constexpr std::underlying_type_t<::McuCommand> max = ::McuCommand::BUSY;

  static constexpr bool is_valid(std::underlying_type_t<::McuCommand> value) {
    if (value < min || value > max) { return false; }
    // <snip>
  }
}

Add a proper validator

We could also generate a proper validator that will work with emboss views:

struct EnumValidator<McuCommand> {
  template <typename ValueType>
  static constexpr bool ValueIsOk(ValueType value) {
    // add some checks that the value would fit in the underlying type
    return EnumTrait<McuCommand>::is_valid(static_cast<std::underlying_type_t<::McuCommand>>(value));
  }
};

Hook the validator in

We could go further and use the validator by default.
Pros: This is the right thing to do
Cons: This might cause runtime assertions for existing code.

Something like:

template <class Storage>
inline typename ::emboss::support::EnumView<
    /**/ ::McuCommand,
    ::emboss::support::FixedSizeViewParameters<24, EnumValidator<McuCommand>>,
    typename ::emboss::support::BitBlock</**/::emboss::support::LittleEndianByteOrderer<typename Storage::template OffsetStorageType</**/0, 6>>, 24>>

 GenericMcuMessageHeaderView<Storage>::reason()
    const;