Obsolete member accessing another obsolete member

If an obsolete member accesses another obsolete member, Microsoft's C# compilers do not issue the warning required by [C# 6.0 §21.5.4](https://github.com/dotnet/csharpstandard/blob/V6-standard/standard/attributes.md#2154-the-obsolete-attribute). Should this suppression be part of the C# standard? If not, does the standard allow it as an extension (which would then have to be defined in the conformance document)?

For example:

```csharp
static class Demo
{
[System.Obsolete]
static void M() {}

[System.Obsolete]
static void N()
{
M(); // no warning about accessing obsolete M, because N is also obsolete
}

static void O()
{
N(); // warning issued
}
}
```

At least the following compiler versions behave this way:

* Microsoft (R) Visual C# 2005 Compiler version 8.00.50727.9149\
for Microsoft (R) Windows (R) 2005 Framework version 2.0.50727\
Copyright (C) Microsoft Corporation 2001-2005. All rights reserved.
* Microsoft (R) Visual C# Compiler version 4.8.4084.0\
for C# 5\
Copyright (C) Microsoft Corporation. All rights reserved.
* Microsoft (R) Visual C# Compiler version 2.10.0.0 (b9fb1610)\
Copyright (C) Microsoft Corporation. All rights reserved