Lambdas + custom delegate types + type inference = breaking change in C# 10

[controlflow]

Version Used:

.NET 6.0 SDK RTM

Steps to Reproduce:

Compile the following code:

using System;

DataRules.AddRule(new Constant<MyDelegate>(), () => Function); // Error
DataRules.AddRule(new Constant<Func<string, int>>(), () => Function); // OK
DataRules.AddRule<MyDelegate>(new Constant<MyDelegate>(), () => Function); // OK

int Function(string s) => s.Length;

static class DataRules
{
  public static void AddRule<T>(Constant<T> constant, Func<T> factory) { }
  public static void AddRule<T>(Constant<T> constant, T value) { }
}

class Constant<T> { }
delegate int MyDelegate(string s);

Expected Behavior:

This code compiles in C# 9.0/.NET 5.0 SDK, all invocations are bound to AddRule<T>(Constant<T> constant, Func<T> factory) overload. I expect the same behavior in C# 10/.NET 6.0 SDK

Actual Behavior:

Failed build in C# 10/.NET 6.0 SDK:

CS1593 Delegate 'MyDelegate' does not take 0 arguments

If I understand correctly, the compiler should prefer overloads that don't infer "function type" for the type arguments.

[controlflow]

Probably related to #57713 (comment)

[svick]

This looks like a duplicate of #57630, which has already been fixed. Indeed, when I build your code using a daily build (Microsoft.Net.Compilers.Toolset version 4.0.1-1.21569.2), it compiles successfully.

Edit: I see you already found the PR fixing that issue.

[cston]

Thanks for reporting this issue @controlflow.

As @svick mentioned, this was also reported in #57630 and has been fixed.

[jcouv]

@cston Should this issue be closed then?