CA1851: fails to detect multiple enumeration in certain scenarios

[hchamre]

Analyzer

Diagnostic ID: CA1851: Possible multiple enumerations of IEnumerable collection

Analyzer source

NuGet Package: Microsoft.CodeAnalysis.NetAnalyzers

Version: 8.0.0

Describe the bug

Several cases of multiple enumeration are not detected by the analyzer.

Steps To Reproduce

Preparation:

• Create an empty C# project.

• Enable NetAnalyzers.

• Enable CA1851 in EditorConfig by setting severity to error or warning.

• Use the following code snippets:

  1. Multiple enumeration with null-conditional ?.:

     IEnumerable<string>? values = SomeMethodReturning<IEnumerable<string>?>();
     int? x = values?.Count() + values?.Count();

  2. Multiple enumeration with reassignment in conditional:

     IEnumerable<int> values = SomeMethodReturning<IEnumerable<int>>();
     if (values.Count() == 0)
     {
         values = new List<int> { 123 };
     }
     int x = values.Count();

  3. Multiple enumeration of IQueryable via IEnumerable:

     IEnumerable<int> values = SomeMethodReturning<IQueryable<int>>();
     int x = values.Count() + values.Count();

Expected behavior

Each of the above snippets should generate a "multiple enumeration" diagnostic. Here is my reasoning:

1. Either values is null and then it is never enumerated or values is not null and is enumerated twice.
2. If values.Count() != 0, then values is enumerated twice.
3. values is enumerated twice.

Actual behavior

None of the snippets generate diagnostics.

Additional context

Rider has a similar inspection which is able to flag 1) and 2), but not 3).