Struct types with custom equality operators doesn't report CS0019 error when compared with null.

[zlatanov]

Version Used:
Visual Studio 15.3.1

Steps to Reproduce:

1. Create a dummy struct Value such as:

struct Value
{
    public static Boolean operator ==( Value left, Value right ) => true;
    public static Boolean operator !=( Value left, Value right ) => false;
}

2. Then the following Main procedure:

static void Main()
{
    var value = new Value();

    // No Error CS0019  Operator '!=' cannot be applied to operands of type 'Value' and '<null>'
    if ( value == null )
    {
        // This line seems OK to the analyzer, but even the compiler is removing it.
        // Setting up break point here is removed to the next line when debugging.
        Console.WriteLine( "This is not even compiled." );
     }
}

Expected Behavior:
The compiler or analyzer should issue CS0019 error.

Actual Behavior:
The code is built without any error and the code block inside the if is omitted.

This really stung me in one of my projects where I had to make what once was a class to a struct and I expected that the compiler will warn me of places where I compared it to null or not null.

If the value type doesn't have the custom operators == and != the code doesn't compile and the expected error CS0019 is given.

[jcouv]

@zlatanov Just to confirm, you don't think this is a regression, correct?

I tried this on csc.exe version 1.3.2 (which shipped in the latest update of Visual Studio 2015) and also the previous/native csc.exe compiler and I get the same behavior.

[zlatanov]

@jcouv No, I don't think this is a regression. Just never had the case before where something was class that I needed it to be a struct ( with custom operators ) to notice it.

[jcouv]

Thanks for the clarification. Updated the title.
Adding a new warning is a breaking change, and so needs to be considered as part of warning waves (see #1580 for broader workitem).

[jcouv]

Now that we have warning waves, we should be able to warn for this. I'll check with Neal when he's back.

[gafter]

This is already implemented in C# 9 at warning level 5. See CS8073 in https://github.com/dotnet/roslyn/blob/master/docs/compilers/CSharp/Warnversion%20Warning%20Waves.md