Compiler error (Init-only setters) when using LangVersion 8.0 in a .NET Standard 2.0 library

Description (optional)

I got compiler error CS8400 Feature 'init-only setters' is not available in C# 8.0. Please use language version 9.0 or greater. when compiling a .NET Standard 2.0 class library with C# 8.0 as the LangVersion. Upgrading to 9.0 fixed the issue but should not be nesessary.

Reproduction Steps

Create a Class Library using the following .csproj content and this class Product.cs

<Project Sdk="Microsoft.NET.Sdk">
	<PropertyGroup>
		<TargetFramework>netstandard2.0</TargetFramework>
		<LangVersion>8.0</LangVersion>
		<Nullable>enable</Nullable>
	</PropertyGroup>
	<ItemGroup>
	  <PackageReference Include="PolySharp" Version="1.6.0" />
	</ItemGroup>
</Project>

using System.Diagnostics.CodeAnalysis;

namespace ClassLibrary19 {
    public class Product {

        [AllowNull]
        public string ProductName { get; set; }
    }
}

Expected Behavior

No compiler error

Actual Behavior

error CS8400

System info

This section should contain useful info such as:

• PolySharp NuGet version 1.6

Additional context (optional)

Any additional info that might be useful to resolve the issue.

[sonnemaf]

Thank you! It is really great to have this. I was using this Nullable package before. But PolySharp has much more. Happy!

[sonnemaf]

Thanks for this tip. I will set the PrivateAssets=“all”.