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).