Fix S4070 FP: Raised on flagged enum

[muffeldsound]

Description

Get the S4070 on flaged enum

Repro steps

[Flags]
public enum Failing
{
	None = 0,
            MyTest = 1 << 31
    }

Expected behavior

No S4070

Actual behavior

Get the S4070 on flaged enum

Related information

• Visual Studio version

[cristian-ambrosini-sonarsource]

Hi @muffeldsound!
Thanks for reporting this.
After reviewing the issue, I can confirm that it is a false positive. I will provide a reproducer shortly to demonstrate the problem.
The root cause of this issue lies in the way we retrieve the value from the IFieldSymbol in the GetEnumValueOrDefault method (see here). The method fails with bitwise operations since they are not constant values.

[costin-zaharia-sonarsource]

The issue is raised since we consider negative values as noncompliant. We even have a test case for this. See:

sonar-dotnet/analyzers/tests/SonarAnalyzer.UnitTest/TestCases/DoNotMarkEnumsWithFlags.cs

Lines 61 to 66 in fba37e8

	[Flags]
	public enum Color5 : int // Noncompliant
	{
	Value = -3, // Secondary
	Other = -4 // Secondary
	}

In this example 1 << 31 is outside the Int32 range (int is signed), the default base type for enums. Due to this, it overflows.