XML doc comment on extended partial method without body

[KalleOlaviNiemitalo]

Inspired by dotnet/runtime#44969 (comment), I tried placing XML documentation comments in a separate file and hooking them up with extended partial methods (#3301, Docs). However, the C# compiler in .NET 5.0 SDK appears to ignore the XML documentation comment on the partial method declaration that does not have a body. Is this by design?

partialdoc.csproj:

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <TargetFramework>net5.0</TargetFramework>
    <GenerateDocumentationFile>true</GenerateDocumentationFile>
  </PropertyGroup>

</Project>

Class1.cs:

using System;

namespace partialdoc
{
    public partial class Class1
    {
        public partial void Method() => throw new NotImplementedException();
    }
}

Class1.docs.cs:

namespace partialdoc
{
    /// <summary>Class documentation.</summary>
    partial class Class1
    {
        /// <summary>Method documentation.</summary>
        public partial void Method();
    }
}

Build:

$ dotnet build
Microsoft (R) Build Engine version 16.8.0+126527ff1 for .NET
Copyright (C) Microsoft Corporation. All rights reserved.

  Determining projects to restore...
  All projects are up-to-date for restore.
C:\redacted\partialdoc\Class1.cs(7,29): warning CS1591: Missing XML comment for publicly visible type or member 'Class1.Method()' [C:\redacted\partialdoc\partialdoc.csproj]
  partialdoc -> C:\redacted\partialdoc\bin\Debug\net5.0\partialdoc.dll

Build succeeded.

C:\redacted\partialdoc\Class1.cs(7,29): warning CS1591: Missing XML comment for publicly visible type or member 'Class1.Method()' [C:\redacted\partialdoc\partialdoc.csproj]
    1 Warning(s)
    0 Error(s)

Time Elapsed 00:00:01.44

The resulting bin/Debug/net5.0/partialdoc.xml omits the "Method documentation" that was in Class1.docs.cs:

<?xml version="1.0"?>
<doc>
    <assembly>
        <name>partialdoc</name>
    </assembly>
    <members>
        <member name="T:partialdoc.Class1">
            <summary>Class documentation.</summary>
        </member>
    </members>
</doc>

[KalleOlaviNiemitalo]

This works differently in .NET SDK 6.0.300. I now get:

<?xml version="1.0"?>
<doc>
    <assembly>
        <name>partialdoc</name>
    </assembly>
    <members>
        <member name="T:partialdoc.Class1">
            <summary>Class documentation.</summary>
        </member>
        <member name="M:partialdoc.Class1.Method">
            <summary>Method documentation.</summary>
        </member>
    </members>
</doc>

even when targeting .NET 5.0.

#6014 is related.