dotnet build error

[Varorbc]

Steps to reproduce

1.

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

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>netcoreapp2.2</TargetFramework>
  </PropertyGroup>

</Project>

using System;

namespace ConsoleApp1
{
    public class Program
    {
        public static void Main(string[] args)
        {
            Console.WriteLine("Hello World!");
            if (GetT<Program>() is null)
            {
                Console.WriteLine("null");
            }
        }

        public static T GetT<T>() where T : new()
        {
            var t = new T();
            if (t is null)
            {
                throw new Exception();
            }

            return t;
        }
    }
}

2. dotnet build dotnet build ConsoleApp1.csproj error

3.build by VS2019 16.0.0 RC.3 is success

Expected behavior

dotnet build success

Actual behavior

dotnet build error

Program.cs(19,22): error CS0403: 无法将 null 转换为类型参数“T”，因为它可能是不可以为 null 的值类型。请考虑改用“default(T)”

Environment data

dotnet --info output:

.NET Core SDK（反映任何 global.json）:
Version: 2.2.105
Commit: 7cecb35

运行时环境:
OS Name: Windows
OS Version: 10.0.17763
OS Platform: Windows
RID: win10-x64
Base Path: C:\Program Files\dotnet\sdk\2.2.105\

Host (useful for support):
Version: 2.2.3
Commit: 6b8ad509b6

.NET Core SDKs installed:
2.2.105 [C:\Program Files\dotnet\sdk]

.NET Core runtimes installed:
Microsoft.AspNetCore.All 2.2.3 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.App 2.2.3 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.NETCore.App 2.2.3 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]

To install additional .NET Core runtimes or SDKs:
https://aka.ms/dotnet-download

[peterhuene]

I would expect the need for a class constraint on the generic parameter T for Get<T> to get past the CS0403 error.

However, with a 3.0 SDK, I can't seem to get it to error with a CS0403 regardless of the LangVersion I use (7.0+ since this example uses a pattern).

@jaredpar is this some subtle behavior difference between the compilers that ship with 2.2 and 3.0?

Regardless, this probably belongs as a Roslyn issue, but I'd like to discuss it before moving.

[jaredpar]

@cston can confirm but believe this is a deliberate change between C# 7.3 and C# 8.0 (the latter allows the code now).

[Varorbc]

@jaredpar I set the language version to 7.0 in vs 2019 and still compiled successfully.

[jaredpar]

@Varorbc I noticed that as well and that appears to be a bug. If you actually use a 7.0 compiler though you'll see it doesn't compile.

[Varorbc]

@jaredpar Yes, I don't know exactly what version is used in the project system.@davkean Can you tell us?

[cston]

Adding @gafter for the compiler change since t is null is bound as an is pattern now.

[jcouv]

I think the change is deliberate and comes from dotnet/roslyn#25995 ("recursive-patterns(18): Permit a constant pattern to be used with an open type as input")

Here's the championed issue: dotnet/csharplang#1284

[peterhuene]

@jcouv Thanks for the info! However, should -langversion:7.0 with a Dev16 compiler emit CS0403 for the code above or not?

If the answer is "no", then I think we can close this by design. If the answer is "yes", then we should transfer this to issue to Roslyn (with some changes, since this is expected behavior with a .NET Core 2.2 SDK, but it should be possible to get the error with a LangVersion set to 7.0 with a 3.0 SDK).

[peterhuene]

Ping @jcouv for follow-up question above.

[jcouv]

I checked with @gafter who said this was a new feature (as opposed to a bug fix). That means that t is null should produce a diagnostic when LangVersion is < 8. That diagnostic should indicate that upgrading the LangVersion would make the code compile.

[jcouv]

Moving the issue over to Roslyn repo: dotnet/roslyn#34678