feat(C++): Support underlying_type for union

[sssooonnnggg]

No description provided.

[dbaileychess]

The last thing I think is evolution test. It should be invalid to change the backing type of a union. Can you add this as well?

[sssooonnnggg]

@dbaileychess Hi, I've added evolution tests for enum / union underlying type changes

[dbaileychess]

Great! Thanks for the multiple edits.

[jmillan]

Hi @sssooonnnggg , would you please briefly comment what is this change about? I see no description at all in the PR.

Thanks

[sssooonnnggg]

Hi @jmillan , the default union implementation use a single byte to store union associative enums, which limits union count must smaller than 256 , this feature allow union to specify an underlying type to store union associative enums with arbitrary integer type.

For example：

union LargeUnion: int { A, B, C, ...}