Microsoft.NET.StringTools hijacks System.MemoryExtensions in .NET Framework <4.7.2

[RussKie]

Issue Description

#5663 introduced public extensions methods:

msbuild/src/StringTools/InternableString.Simple.cs

Lines 7 to 23 in 5a635cf

	namespace System
	{
	/// <summary>
	/// A bare minimum and inefficient version of MemoryExtensions as provided in System.Memory on .NET 4.5.
	/// </summary>
	public static class MemoryExtensions
	{
	public static string AsSpan<T>(this T[] array, int start, int length)
	{
	if (array is char[] charArray)
	{
	return new string(charArray, start, length);
	}
	throw new ArgumentException(nameof(array));
	}
	}
	}

This is a really bad idea... I have a project that is targeting net462 and I get this error because of this code:

error CS0433: The type 'MemoryExtensions' exists in both 'Microsoft.NET.StringTools.net35, Version=1.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' and 'System.Memory, Version=4.0.1.1, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51'

The trouble is that we have control over the dependency tree:

...and the code that's failing is source generated.

Steps to Reproduce

1. Run dotnet new console
2. Update the project file as follows:

   <Project Sdk="Microsoft.NET.Sdk">
   
     <PropertyGroup>
       <OutputType>Exe</OutputType>
       <TargetFramework>net462</TargetFramework>
       <LangVersion>latest</LangVersion>
     </PropertyGroup>
     <ItemGroup>
       <PackageReference Include="MessagePack" Version="2.4.35" />
       <PackageReference Include="Microsoft.AspNetCore.Mvc.Testing" Version="2.2.0" />
       <PackageReference Include="System.Runtime.CompilerServices.Unsafe" Version="6.0.0" />
     </ItemGroup>
   
   </Project>

3. Add the following line to Program.cs:

   MemoryExtensions.AsSpan("bla");

[KalleOlaviNiemitalo]

Duplicate of #8295?

[RussKie]

Yes, it is. I should have searched, shouldn't have I? :)