NBMsgPack010: [KnownSubType] should specify an assignable type

@Nerdbank.MessagePack.KnownSubTypeAttribute should specify a type that is assignable to the type the attribute is applied to.

Learn more about subtype unions.

Example violations

[KnownSubType<DerivedType>(1)] // DerivedType is not in fact derived from BaseType
class BaseType
{
}

[GenerateShape]
class DerivedType
{
}

Resolution

Remove the attribute or specify a type that is actually derived from the applied type.

In the fixed code below, the DerivedType is actually fixed to derive from BaseType.

[KnownSubType<DerivedType>(1)]
class BaseType
{
}

[GenerateShape]
class DerivedType : BaseType
{
}