.NET 6 RC 1: changes to overload resolution? Breaks NUnit?

[jonathanpeppers]

Version Used:

> dotnet --version
6.0.100-rc.1.21452.4

Steps to Reproduce:

1. Unzip this somewhere: repro.zip
2. dotnet build fails with:

Program.cs(3,8): error CS0121: The call is ambiguous between the following methods or properties: 'Assert.That(TestDelegate, IResolveConstraint)' and 'Assert.That<TActual>(TActual, IResolveConstraint)'

3. If you uncomment <LangVersion>9.0</LangVersion> it works.

The problematic method is NUnit:

Assert.That(() => { }, Throws.Nothing);

We hit this in .NET MAUI here:

dotnet/maui#2369

The breaking change might be expected. The part that is concerning, is we had some unit tests fail at runtime due to a similar issue, where changing LangVersion=9.0 fixed the test failure:

https://github.com/dotnet/maui/blob/f22a61ac027bb115a115a0f493c97ba0c98fa3e8/src/Core/tests/UnitTests/LifecycleEvents/LifecycleEventsTests.cs#L119-L136

I thought it might be worth checking into, thanks!

Expected Behavior:

NUnit Assert.That(() => { }, Throws.Nothing); compiles in C# 10?

Actual Behavior:

NUnit Assert.That(() => { }, Throws.Nothing); doesn't compile under C# 10.

[cston]

Thanks @jonathanpeppers for reporting these cases.

The part that is concerning, is we had some unit tests fail at runtime due to a similar issue, where changing LangVersion=9.0 fixed the test failure:

https://github.com/dotnet/maui/blob/f22a61ac027bb115a115a0f493c97ba0c98fa3e8/src/Core/tests/UnitTests/LifecycleEvents/LifecycleEventsTests.cs#L119-L136

It looks like the lambda expressions in the test may be binding to the generic overload with TDelegate = Func<int> rather than the overload with Action.

[jonathanpeppers]

Repro steps for the second issue, this failing test.

1. Clone dotnet/maui
2. Checkout the release/6.0.1xx-rc1 branch
3. dotnet build src/DotNet/DotNet.csproj - will create a .NET SDK in ./bin/dotnet/ and install optional workloads
4. ./bin/dotnet/dotnet test src/Core/tests/UnitTests/Core.UnitTests-net6.csproj -p:LangVersion=10.0

This actually shows 3 test failures, but they all appear to be the same thing. You can drop -p:LangVersion and the problem goes away, because we currently have LangVersion=9.0 in the .csproj file.