CSC doesn't care if out parameter not assigned

[carlreinke]

Version Used: Microsoft (R) Visual C# Compiler version 2.6.0.62329 (5429b35)

Steps to Reproduce:

c:\temp>type Program.cs
class C
{
    static bool M( out System.ReadOnlyMemory<byte> x )
    {
        return false;  // No error here!?
    }
}

c:\temp>csc.exe /reference:"C:\Program Files\dotnet\sdk\NuGetFallbackFolder\microsoft.netcore.app\2.0.0\ref\netcoreapp2.0\mscorlib.dll" /reference:"C:\Program Files\dotnet\sdk\NuGetFallbackFolder\microsoft.netcore.app\2.0.0\ref\netcoreapp2.0\netstandard.dll" /reference:C:\Users\User\.nuget\packages\system.memory\4.5.0-preview1-26011-01\ref\netstandard2.0\System.Memory.dll /reference:"C:\Program Files\dotnet\sdk\NuGetFallbackFolder\microsoft.netcore.app\2.0.0\ref\netcoreapp2.0\System.Runtime.dll" /out:Test.dll /target:library Program.cs
Microsoft (R) Visual C# Compiler version 2.6.0.62329 (5429b35d)
Copyright (C) Microsoft Corporation. All rights reserved.


c:\temp>

Expected Behavior: Show error message and fail to compile.

Actual Behavior: Compilation succeeds.

[gafter]

Please check to see what fields, if any, are contained in System.Collections.Generic.KeyValuePair inside of C:\Program Files\dotnet\sdk\NuGetFallbackFolder\microsoft.netcore.app\2.0.0\ref\netcoreapp2.0\System.Runtime.dll. If it contains no fields in that reference assembly, then the compiler is correct. In that case the out parameter is definitely assigned because all (zero) of its fields are definitely assigned.

[gafter]

To clarify, if that is the case then it is a bug in the reference assembly.

[carlreinke]

Indeed, there are no fields in KeyValuePair<TKey, TValue> in the reference assembly. I replaced the original comment with a different example. ReadOnlyMemory<T> does have fields.

[carlreinke]

Err, I guess the reference assembly for ReadOnlyMemory<T> doesn't have fields either... the assembly that gets used when it actually runs has fields; otherwise it wouldn't work.

Why are there no fields in the reference assemblies?

(IntelliSense/MSBuild seem to also use the ref assembly, so when I'm building in Visual Studio, I don't get any error message about the out parameter not being assigned there either.)

[gafter]

That is a bug. We’ll move this bug to an appropriate repository.

[tannergooding]

@gafter, I believe the issue has already been dealt with on the CoreFX side: dotnet/corefx#26286 and dotnet/buildtools#1859.

However, it was only merged 15 days ago, so nothing has shipped with the fix yet.

[gafter]

@tannergooding I don't see anything specifically changing KeyValuePair's reference assembly. Isn't its reference assembly produced based on hand-written source?

[gafter]

Issue moved to dotnet/corefx #26624 via ZenHub

[tannergooding]

It looks like it has already been fixed, at least somewhere: https://github.com/dotnet/corefx/blob/a3986843153e27e92c89907f58046bb37da0f9eb/src/System.Memory/ref/System.Memory.cs#L108

[gafter]

And the fix for KeyValuePair?

[tannergooding]

I wasn't able to find the contract assembly for KVP and commented as such on the new issue (forgot to comment as such here as well).

[tannergooding]

Nevermind, it's in System.Runtime and is also correct: https://github.com/dotnet/corefx/blob/master/src/System.Runtime/ref/System.Runtime.cs#L3907

(GitHub search wouldn't find it, git grep did).