Add MA0201 for zero-valued enum flag checks

README.md

@@ -219,6 +219,7 @@ If you are already using other analyzers, you can check [which rules are duplica
 |[MA0198](https://github.com/meziantou/Meziantou.Analyzer/blob/main/docs/Rules/MA0198.md)|Design|Specify cref for ambiguous inheritdoc on types|⚠️|✔️|✔️|
 |[MA0199](https://github.com/meziantou/Meziantou.Analyzer/blob/main/docs/Rules/MA0199.md)|Design|Do not use inheritdoc on types without inheritance source|⚠️|✔️|❌|
 |[MA0200](https://github.com/meziantou/Meziantou.Analyzer/blob/main/docs/Rules/MA0200.md)|Usage|Do not use empty property patterns with non-nullable value types|ℹ️|✔️|✔️|
+|[MA0201](https://github.com/meziantou/Meziantou.Analyzer/blob/main/docs/Rules/MA0201.md)|Usage|Do not use zero-valued enum flags in flag checks|⚠️|✔️|❌|
 
 <!-- rules -->
 

docs/README.md

@@ -199,6 +199,7 @@
 |[MA0198](https://github.com/meziantou/Meziantou.Analyzer/blob/main/docs/Rules/MA0198.md)|Design|Specify cref for ambiguous inheritdoc on types|<span title='Warning'>⚠️</span>|✔️|✔️|
 |[MA0199](https://github.com/meziantou/Meziantou.Analyzer/blob/main/docs/Rules/MA0199.md)|Design|Do not use inheritdoc on types without inheritance source|<span title='Warning'>⚠️</span>|✔️|❌|
 |[MA0200](https://github.com/meziantou/Meziantou.Analyzer/blob/main/docs/Rules/MA0200.md)|Usage|Do not use empty property patterns with non-nullable value types|<span title='Info'>ℹ️</span>|✔️|✔️|
+|[MA0201](https://github.com/meziantou/Meziantou.Analyzer/blob/main/docs/Rules/MA0201.md)|Usage|Do not use zero-valued enum flags in flag checks|<span title='Warning'>⚠️</span>|✔️|❌|
 
 |Id|Suppressed rule|Justification|
 |--|---------------|-------------|
@@ -811,6 +812,9 @@ dotnet_diagnostic.MA0199.severity = warning
 
 # MA0200: Do not use empty property patterns with non-nullable value types
 dotnet_diagnostic.MA0200.severity = suggestion
+
+# MA0201: Do not use zero-valued enum flags in flag checks
+dotnet_diagnostic.MA0201.severity = warning
 ```
 
 # .editorconfig - all rules disabled
@@ -1409,4 +1413,7 @@ dotnet_diagnostic.MA0199.severity = none
 
 # MA0200: Do not use empty property patterns with non-nullable value types
 dotnet_diagnostic.MA0200.severity = none
+
+# MA0201: Do not use zero-valued enum flags in flag checks
+dotnet_diagnostic.MA0201.severity = none
 ```

docs/Rules/MA0192.md

@@ -18,6 +18,8 @@ This rule reports patterns such as:
 
 For comparisons against `0`, the enum member used in the bitwise `&` must be a single-bit value (for example `1`, `2`, `4`, `8`, ...). Combined values are ignored.
 
+Zero-valued enum members are not reported by this rule. They are covered by [MA0201](https://github.com/meziantou/Meziantou.Analyzer/blob/main/docs/Rules/MA0201.md).
+
 ## Non-compliant code
 
 ````csharp

docs/Rules/MA0201.md

@@ -0,0 +1,46 @@
+# MA0201 - Do not use zero-valued enum flags in flag checks
+<!-- sources -->
+Source: [UseHasFlagMethodAnalyzer.cs](https://github.com/meziantou/Meziantou.Analyzer/blob/main/src/Meziantou.Analyzer/Rules/UseHasFlagMethodAnalyzer.cs)
+<!-- sources -->
+
+Using a zero-valued enum member (`None = 0`) as a flag to test always produces a constant result.
+
+This rule reports patterns such as:
+
+- `(value & MyEnum.None) == MyEnum.None` (always `true`)
+- `(value & MyEnum.None) != MyEnum.None` (always `false`)
+- `(value & MyEnum.None) is MyEnum.None` (always `true`)
+- `(value & MyEnum.None) is not MyEnum.None` (always `false`)
+- `value.HasFlag(MyEnum.None)` (always `true`)
+
+## Non-compliant code
+
+````csharp
+[System.Flags]
+enum MyEnum
+{
+    None = 0,
+    Flag1 = 1,
+}
+
+bool M(MyEnum value)
+{
+    return (value & MyEnum.None) == MyEnum.None;
+}
+````
+
+## Compliant code
+
+````csharp
+[System.Flags]
+enum MyEnum
+{
+    None = 0,
+    Flag1 = 1,
+}
+
+bool M(MyEnum value)
+{
+    return value.HasFlag(MyEnum.Flag1);
+}
+````

src/Meziantou.Analyzer.Pack/configuration/all-errors.editorconfig

@@ -595,3 +595,6 @@ dotnet_diagnostic.MA0199.severity = error
 
 # MA0200: Do not use empty property patterns with non-nullable value types
 dotnet_diagnostic.MA0200.severity = error
+
+# MA0201: Do not use zero-valued enum flags in flag checks
+dotnet_diagnostic.MA0201.severity = error

src/Meziantou.Analyzer.Pack/configuration/all-suggestions.editorconfig

@@ -595,3 +595,6 @@ dotnet_diagnostic.MA0199.severity = suggestion
 
 # MA0200: Do not use empty property patterns with non-nullable value types
 dotnet_diagnostic.MA0200.severity = suggestion
+
+# MA0201: Do not use zero-valued enum flags in flag checks
+dotnet_diagnostic.MA0201.severity = suggestion

src/Meziantou.Analyzer.Pack/configuration/all-warnings.editorconfig

@@ -595,3 +595,6 @@ dotnet_diagnostic.MA0199.severity = warning
 
 # MA0200: Do not use empty property patterns with non-nullable value types
 dotnet_diagnostic.MA0200.severity = warning
+
+# MA0201: Do not use zero-valued enum flags in flag checks
+dotnet_diagnostic.MA0201.severity = warning

src/Meziantou.Analyzer.Pack/configuration/default.editorconfig

@@ -595,3 +595,6 @@ dotnet_diagnostic.MA0199.severity = warning
 
 # MA0200: Do not use empty property patterns with non-nullable value types
 dotnet_diagnostic.MA0200.severity = suggestion
+
+# MA0201: Do not use zero-valued enum flags in flag checks
+dotnet_diagnostic.MA0201.severity = warning

src/Meziantou.Analyzer.Pack/configuration/none.editorconfig

@@ -595,3 +595,6 @@ dotnet_diagnostic.MA0199.severity = none
 
 # MA0200: Do not use empty property patterns with non-nullable value types
 dotnet_diagnostic.MA0200.severity = none
+
+# MA0201: Do not use zero-valued enum flags in flag checks
+dotnet_diagnostic.MA0201.severity = none

src/Meziantou.Analyzer/RuleIdentifiers.cs

@@ -200,6 +200,7 @@ internal static class RuleIdentifiers
     public const string InheritdocShouldNotBeAmbiguousOnTypes = "MA0198";
     public const string InheritdocShouldHaveSourceOnTypes = "MA0199";
     public const string DoNotUseEmptyPropertyPatternOnNonNullableValueType = "MA0200";
+    public const string DoNotUseZeroValuedEnumFlagsInFlagChecks = "MA0201";
 
     public static string GetHelpUri(string identifier)
     {