Fix S3655 FN: VB implicit conversions

[Tim-Pohlmann]

Follow up to #6795

[antonioaversa]

I think there is a problem on this extension of the ShouldExecute: we are now finding false positives every time a = Nothing is made.

When I paste the following example

Sub Assignment2(nullable As Integer?)
        If nullable = Nothing Then
            Dim x As Integer = nullable     ' Noncompliant
End If

into a VB.NET Project or LINQPad I get the following warning from Roslyn: Warning BC42037 This expression will always evaluate to Nothing (due to null propagation from the equals operator). To check if the value is null consider using 'Is Nothing'.

nullable = Nothing always evaluates to Nothing, which is implicitly converted to the Boolean value False. As a result of that, Dim x As Integer = nullable is never executed, and the non-compliance is in this case a false positive.

The same applies to all cases below. I think the right way of extending the rule was to capture Is Nothing instead, which is the VB.NET equivalent of == null.

[antonioaversa]

LGTM

I would just expand tests to include <> Nothing.

[antonioaversa]

I would add a test with <> Nothing (which has a behavior very similar to = Nothing:

Function ReturnStatement(nullable As Integer?) As Integer
    If nullable <> Nothing Then
        Return nullable ' Noncompliant FP
    End If
End Function

And also the following case, which shows how = and <> break tautology w.r.t. OrElse:

Function ReturnStatement(nullable As Integer?) As Integer
    If (nullable <> Nothing) OrElse (nullable = Nothing) Then
        Return nullable ' Noncompliant
    End If
End Function

Of course you could "exploit" the fact that = and <> lift Nothing to get all sort of ternary logic behaviors. I would let you decide how deep you think it's worth exploring this path.

[pavel-mikula-sonarsource]

LGTM

[pavel-mikula-sonarsource]

I would repeat this unusual comment in the testcases below as well.

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