In the example code there is "using Length = long;", but long is not portable, shouldn't be this be int32_t?

Spec says:

If the union has a default case, the default constructor shall initialize the discriminator, and the selected
member field following the initialization rules described in Clause 7.2.4.3.1. If it does not, the default
constructor shall initialize the union to the first discriminant value specified in the IDL definition.

But when there is an implicit default member, that should be selected, for example, the example below (

  enum DataType
  {
    dtEmpty,
    dtLong,
    dtShort,
    dtString,
    dtPoint,
    dtTrack,
    dtGlobal
  };

  union Data switch (DataType)
    {
      case dtLong: long longData;
      case dtShort: short shortData;
      case dtString: string stringData;
      case dtPoint: string pointData;
      case dtTrack: string trackData;
      case dtGlobal: string globalData;
      // by default (implicit), empty union
    };

The spec gives:

const <MemberType>& <MemberName>() const

But for basic types/int this will result in a warning, for basic/enum it should be

<MemberType> <MemberName>() const

The spec says:

A public constant accessor method with the name of the union member that returns a constant reference to
its value:
const <MemberType>& <MemberName>() const;
Accessing an invalid union member may result in an undefined error.

A language mapping should be precise and very clear to my idea, the user should know what happens if an invalid union member is accessed, this could happen in some cases, the spec should specify which exception will be thrown so that the programmer can react on this, saying it is undefined is not enough.

This should be changed to a "may" or a "could". Shall seems overly prescriptive.

How could a compliant implementation of Equals<T> be distinguished from Equals(Object) from the callers/users perspective?

The annotations should be covered in a separate section of the document and NOT be part of a clean, canonical example.

Suggest:
// IDL
enum E

;

// C#
public enum E

;

Also should enums inherit uint?

Anonymous types should be moved to their own section

Suggest:

//IDL
typedef long myLongArray[2][3];

// C# example

Suggestion is to have one section or appendix where possible naming mappings could be covered.

Change the IDL example naming to match the C# community expectations if that makes it easier to specify

Beside basic types no other canonical example is included. I suggest removing the basic types and having one example.

// IDL
struct foo

;

Something like this.

Types such as sbyte and int32 are included in this section and should not be.

These types should be moved to the extended types section.

The section for these types is missing from the document.

The section that describes the default value does not include a discussion
of what to do when a default is not present. In this case a method called _default or
__default should be defined to set the discriminant to a non-switched field value.
This should be fixed before December.

Every scope containing a constant declaration shall contain a public static partial class.

The "public static partial class" is inconsistent with the examples in 7.2.3.1 above.

The use of "partial" is suspect and cannot span assemblies. It may be useful for forward interface declarations though whose definition appears in another IDL file.