'await' cannot be used in an expression containing the type 'System.ReadOnlySpan<char>'

[majora2007]

Describe the bug
When utilizing System.IO.Abstractions for Path.Join, the following code snippet does not compile on .NET 6, however it does when using System.IO Path.Join.

    var path = System.IO.Abstractions.FileSystem.Path.Join("/config/covers", await GetChapterCoverImageAsync(chapterId));
    public async Task<string> GetChapterCoverImageAsync(int chapterId)
        {
    
            return await _context.Chapter
                .Where(c => c.Id == chapterId)
                .Select(c => c.CoverImage)
                .AsNoTracking()
                .SingleOrDefaultAsync();
        }

public class Chapter {
   public string CoverImage { get; set; }
}

Expected behavior
I expect that the Abstraction works identically to System.IO and it builds and functions correctly.

Additional context
I am using .NET 6 and System.IO.Abstractions 14.0.3

[fgreinacher]

Thanks for reporting @majora2007! Can you share a self-contained reproduction? A tiny snippet like #713 (comment) is totally fine.

[majora2007]

@fgreinacher

I just wrote a small .NET 6 program:

Program.cs:

using AbstractionsReproductionTest;

Console.WriteLine("Hello, World!");

var p = await new GetTest().Path();
Console.Write(p);

GetTest

using System.IO.Abstractions.TestingHelpers;

namespace AbstractionsReproductionTest;

public class GetTest
{
    public async Task<string> Path()
    {
        return new MockFileSystem().Path.Join("/config/covers", await GetPath());
    }

    private async Task<string> GetPath()
    {
        return await Task.Run(() => "Hello");
    } 
}

[fgreinacher]

Thanks a ton for sharing the repro!

I could reproduce and it seems the root cause is that we're missing the string overloads for Path.Join(), causing overload resolution to pick the ReadOnlySpan<char> overloads as next best candidates.

To fix this we need to add the missing overloads. Happy to accept a PR, otherwise I'll take care in the next couple of days.