Language changes are breaking real-world use of dynamic

[stephentoub]

Version Used:
Version 17.10.0 Preview 3.0 [34721.271.main]

Steps to Reproduce:

using System.Text.Json;

public class C
{
    public static C M(IFoo foo, dynamic value)
    {
        var result = foo.Bar("name", value);
        return JsonSerializer.Deserialize<C>(result);
    }
}

public interface IFoo
{
    object Bar(string name, object value);
}

Expected Behavior:
Previously this compiled fine.
SharpLab

Actual Behavior:
Now it fails to compile:

error CS1503: Argument 1: cannot convert from 'object' to 'System.IO.Stream'

After the recent changes around dynamic, the type of result is now object whereas previously it was dynamic.

[jcouv:] Relates to #71421

[mattheww-freshview]

Will this also fix the fact that old versions of VS will compile https://dotnetfiddle.net/OdQkAQ as a legacy .NET Framework 4.8 console app but Version 17.10.0 Preview 4.0 will not?

Specifically

var f = parameter.LastModifiedDate != null ? DateTime.Parse(parameter.LastModifiedDate) : null;
DateTime? f1 = parameter.LastModifiedDate != null ? DateTime.Parse(parameter.LastModifiedDate) : null;

used to compile, now it doesn't (CS8957) if parameter is dynamic.

[AlekseyTs]

Will this also fix the fact that old versions of VS will compile https://dotnetfiddle.net/OdQkAQ as a legacy .NET Framework 4.8 console app but Version 17.10.0 Preview 4.0 will not?

It is not clear to me how this is related to dynamic invocations. If it is, more details might help to give the answer.

[mattheww-freshview]

Specifically

var f = parameter.LastModifiedDate != null ? DateTime.Parse(parameter.LastModifiedDate) : null;
DateTime? f1 = parameter.LastModifiedDate != null ? DateTime.Parse(parameter.LastModifiedDate) : null;

used to compile, now it doesn't (CS8957) if parameter is dynamic and using Visual Studio Version 17.10.0 Preview 4.0.

To repro - copy and paste the code from the dotnetfiddle into Visual Studio Version 17.10.0 Preview 4.0 in a .NET 4.8 Console app and try and run it. It will fail. It works fine on older compilers (e.g. dotnetfiddle, VS 17.9.0) and earlier VS previews (not sure how far back though).

I suppose I am trying to clarify if this is a known bug or not. Apologies if this is unrelated.

[AlekseyTs]

I suppose I am trying to clarify if this is a known bug or not. Apologies if this is unrelated.

This looks like the same issue and I think this scenario will be fixed as well. As a temporary workaround you can convert result of DateTime.Parse calls to dynamic.