Roslyn: Consider 'value is null' instead of 'value == null' for the "Add null check" codefix

[jamesqo]

It's possible that there could be a custom operator defined for ==, which could be slower than a simple pointer comparison. is null reads better and is guaranteed to be just a pointer comparison.

This may warrant looking into the user's code style to see if they already have a lot of == nulls or is nulls, to see which codefix should be offered to maintain consistency.

[MaStr11]

This is minor (and more of a detail that needs to be considered once this gets implemented), but is null isn't a 1:1 replacement for == null. This code

    public static void M<T>(T value)
    {
        if (value is null) { } //error CS0403: Cannot convert null to type parameter 'T' because it could be a non-nullable value type.
    }

does not compile, while value == null would. See also #23581.

[gafter]

We should change the language to permit value is null for an unconstrained generic.

[MaStr11]

For the sake of completeness, there are some more inconsistencies here (but some might be unresolvable):

```C#
public class C {
//unconstraint
public void UnconstraintIs(T v) {
var b = v is null; //error CS0403: Cannot convert null to type parameter 'T' because it could be a non-nullable value type. Consider using 'default(T)' instead.
}
public void UnconstraintNull(T v) {
var b = v == null; // Fine
}

//class
public void RefconstraintIs<T>(T v) where T:class {
    var b = v is null; //Fine
}
public void RefconstraintNull<T>(T v)  where T:class {
    var b = v == null; //Fine
}

//struct
public void ValueconstraintIs<T>(T v) where T:struct {
    var b = v is null; //error CS0403: Cannot convert null to type parameter 'T' because it could be a non-nullable value type. Consider using 'default(T)' instead.
}
public void ValueconstraintNull<T>(T v)  where T:struct {
    var b = v == null; //error CS0019: Operator '==' cannot be applied to operands of type 'T' and '<null>'
}
public void ValueconstraintRefEquals<T>(T v)  where T:struct {
    var b = ReferenceEquals(v, null); // Fine
}

}

[sharplab.io](https://sharplab.io/#v2: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)

```VB
Public Class C
    Public Sub UnconstraintIs(Of T)(v As T)
        Dim flag As Boolean = v Is Nothing
    End Sub

    Public Sub RefconstraintIs(Of T As Class)(v As T)
        Dim flag As Boolean = v Is Nothing
    End Sub

    Public Sub ValueconstraintIs(Of T As Structure)(v As T)
        Dim flag As Boolean = v Is Nothing ' error BC30020: 'Is' operator does not accept operands of type 'T'. Operands must be reference or nullable types.
    End Sub

    Public Sub ValueconstraintReferenceEquals(Of T As Structure)(v As T)
        Dim flag As Boolean = ReferenceEquals(v, Nothing)
    End Sub
End Class

Sharplab.io

[CyrusNajmabadi]

It's possible that there could be a custom operator defined for ==, which could be slower than a simple pointer comparison. is null reads better and is guaranteed to be just a pointer comparison.

Imo, if the user has an overloaded operator == then we should be respecting that. I think using "== null" is appropriate for that feature.

[jinujoseph]

also reported here

[onyxmaster]

Could this be implemented and configured using an option in .editorconfig? Autogenerating code increases productivity (and I'm talking not only about this specific code fix, but also about auto-generated constructors), yet if one wants to enforce the is null/is object pattern for null checks in the codebase, it just won't work.
P.S. https://twitter.com/jaredpar/status/1115019017297596416 @jaredpar