Description of the false positive

In the code below it warns that spanDirective is never read even though the value is passed to a method in which it is read:

if (directiveNode is LineSpanDirectiveTriviaSyntax spanDirective)
{
    return GetLineSpanDirectiveEntry(spanDirective, directiveLineNumber);
}

The compiler transforms the code into this (you can view the IL here)

LineSpanDirectiveTriviaSyntax lineSpanDirectiveTriviaSyntax = directiveNode as LineSpanDirectiveTriviaSyntax;
if (lineSpanDirectiveTriviaSyntax != null)
{
    return GetLineSpanDirectiveEntry(spanDirective, directiveLineNumber);
}

So this is a false positive unless we can prove that there exists no conversion to LineSpanDirectiveTriviaSyntax that can ever succeed.

URL to the alert on the project page on LGTM.com

https://lgtm.com/projects/g/dotnet/roslyn/snapshot/c9e0dd1ec0e440897388bbd55184fb53a555e3a4/files/src/Compilers/CSharp/Portable/Syntax/CSharpLineDirectiveMap.cs?sort=name&dir=ASC&mode=heatmap#xfcee166e229ccb35:1