sdk: Make ImplicitUsings does not work out of the box with .NET Framework project

Apologies if this is not the right repo! Reporting here because there are at least some issues in this repo referencing <ImplicitUsings>

Is your feature request related to a problem? Please describe.

Whenever I create a new class library project, it defaults to setting <ImplicitUsings>enable</ImplicitUsings>. This is good. However, when I multi-target or re-target the project to hit .NET Framework, I get the following compiler error:

The type or namespace name ‘Http’ does not exist in the namespace ‘System.Net’ (are you missing an assembly reference?)

This can be resolved by adding a framework reference, but it would be nice if it “just worked”.

Describe the solution you’d like

I’d like for this to “just work” out of the box. One easy way to do this would be to change the generated usings file (ProjectName.GlobalUsings.g.cs) to look like the following:

// <auto-generated/>
global using global::System;
global using global::System.Collections.Generic;
global using global::System.IO;
global using global::System.Linq;
global using global::System.Net.Http;
global using global::System.Threading;
global using global::System.Threading.Tasks;

// new addition
#if NETFRAMEWORK
namespace System.Net.Http { }
#endif

Potentially we could define all the referenced namespaces to ensure that the auto-generated code never gives an error regardless of what framework is targeted and what references are imported.

Additional context

Example .csproj to reproduce the issue:

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

  <PropertyGroup>
    <TargetFrameworks>net472;netstandard2.0;net6.0</TargetFrameworks>
    <ImplicitUsings>enable</ImplicitUsings>
    <LangVersion>10</LangVersion>
  </PropertyGroup>

</Project>