Csharp 源代码生成器：CS8795分部方法'；Class1.GetS2(字符串)'；必须有实现部分，因为它有可访问性修饰符

我创建了以下项目

ClassLibrary1.csproj个

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

  <PropertyGroup>
    <TargetFramework>netstandard2.1</TargetFramework>
    <Nullable>enable</Nullable>
    <LangVersion>10</LangVersion>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="Microsoft.CodeAnalysis" Version="4.6.0" />
  </ItemGroup>

</Project>

Class1.cs

using System;

namespace ClassLibrary1;

public static partial class Class1
{
    public static string GetS(string s)
    {
        var x = GetS2(s);

        return x ?? throw new InvalidOperationException();
    }

    private static partial string? GetS2(string s); // Error
}

SourceGenerator.cs

using Microsoft.CodeAnalysis;
using System.Text;

namespace ClassLibrary1;

[Generator]
public class SourceGenerator : ISourceGenerator
{
    public void Execute(GeneratorExecutionContext context)
    {
        var sb = new StringBuilder();
        sb.AppendLine("namespace ClassLibrary1;");
        sb.AppendLine("public static partial class Class1");
        sb.AppendLine("{");
        sb.AppendLine("    private static partial string? GetS2(string s)");
        sb.AppendLine("    {");
        sb.AppendLine("        return \"Test\";");
        sb.AppendLine("    }");
        sb.AppendLine("}");

        context.AddSource("Class1.g.cs", sb.ToString());
    }

    public void Initialize(GeneratorInitializationContext context)
    {
    }
}

但是，它得到了以下错误？

错误CS8795分部方法‘Class1.GetS2(字符串)’必须有实现部分，因为它有可访问性修饰符.类库1 C:\source\repos\SourceGeneratorIssue\ClassLibrary1\Class1.cs 14处于活动状态