add EM0015 Open Interface Subtype of a Closed Type Must Be a Case

ExhaustiveMatching.Analyzer.Tests/TypeDeclarationAnalyzerTests.cs

@@ -10,7 +10,7 @@ namespace ExhaustiveMatching.Analyzer.Tests
     public class TypeDeclarationAnalyzerTests : CodeFixVerifier
     {
         [TestMethod]
-        public void SubtypeOfClosedTypeMustBeCase()
+        public void ConcreteSubtypeOfClosedTypeMustBeCase()
         {
             const string test = @"using ExhaustiveMatching;
 namespace TestNamespace
@@ -35,6 +35,32 @@ public class Triangle : Shape { }
             VerifyCSharpDiagnostic(test, expected);
         }
 
+        [TestMethod]
+        public void OpenInterfaceSubtypeOfClosedTypeMustBeCase()
+        {
+            const string test = @"using ExhaustiveMatching;
+namespace TestNamespace
+{
+    [Closed(
+        typeof(ISquare),
+        typeof(ICircle))]
+    public interface IShape { }
+    public interface ISquare : IShape { }
+    public interface ICircle : IShape { }
+    public interface ITriangle : IShape { }
+}";
+
+            var expected = new DiagnosticResult
+            {
+                Id = "EM0015",
+                Message = "Open interface TestNamespace.ITriangle is not a case of its closed supertype: TestNamespace.IShape",
+                Severity = DiagnosticSeverity.Error,
+                Locations = new[] { new DiagnosticResultLocation("Test0.cs", 10, 22, 9) }
+            };
+
+            VerifyCSharpDiagnostic(test, expected);
+        }
+
         [TestMethod]
         public void CaseTypeMustBeSubtype()
         {

ExhaustiveMatching.Analyzer.nuspec

@@ -2,7 +2,7 @@
 <package>
   <metadata>
     <id>ExhaustiveMatching.Analyzer</id>
-    <version>0.3.2</version>
+    <version>0.4.0</version>
     <authors>Jeff Walker</authors>
     <owners>Jeff Walker</owners>
     <license type="expression">BSD-3-Clause</license>

ExhaustiveMatching.Analyzer/ExhaustiveMatchAnalyzer.cs

@@ -40,6 +40,10 @@ public class ExhaustiveMatchAnalyzer : DiagnosticAnalyzer
         private static readonly LocalizableString EM0014Message = LoadString(nameof(Resources.EM0014Message));
         private static readonly LocalizableString EM0014Description = LoadString(Resources.EM0014Description);
 
+        private static readonly LocalizableString EM0015Title = LoadString(nameof(Resources.EM0015Title));
+        private static readonly LocalizableString EM0015Message = LoadString(nameof(Resources.EM0015Message));
+        private static readonly LocalizableString EM0015Description = LoadString(Resources.EM0015Description);
+
         private static readonly LocalizableString EM0100Title = LoadString(nameof(Resources.EM0100Title));
         private static readonly LocalizableString EM0100Message = LoadString(nameof(Resources.EM0100Message));
         private static readonly LocalizableString EM0100Description = LoadString(Resources.EM0100Description);
@@ -86,6 +90,10 @@ public class ExhaustiveMatchAnalyzer : DiagnosticAnalyzer
             new DiagnosticDescriptor("EM0014", EM0014Title, EM0014Message, Category,
                 DiagnosticSeverity.Error, isEnabledByDefault: true, EM0014Description);
 
+        public static readonly DiagnosticDescriptor OpenInterfaceSubtypeMustBeCaseOfClosedType =
+            new DiagnosticDescriptor("EM0015", EM0015Title, EM0015Message, Category,
+                DiagnosticSeverity.Error, isEnabledByDefault: true, EM0015Description);
+
         public static readonly DiagnosticDescriptor WhenGuardNotSupported =
             new DiagnosticDescriptor("EM0100", EM0100Title, EM0100Message, Category,
                 DiagnosticSeverity.Error, isEnabledByDefault: true, EM0100Description);
@@ -105,7 +113,8 @@ public class ExhaustiveMatchAnalyzer : DiagnosticAnalyzer
         public override ImmutableArray<DiagnosticDescriptor> SupportedDiagnostics =>
             ImmutableArray.Create(NotExhaustiveEnumSwitch, NotExhaustiveObjectSwitch,
                 NotExhaustiveNullableEnumSwitch, ConcreteSubtypeMustBeCaseOfClosedType,
-                MustBeDirectSubtype, MustBeSubtype, SubtypeMustBeCovered, WhenGuardNotSupported,
+                MustBeDirectSubtype, MustBeSubtype, SubtypeMustBeCovered,
+                OpenInterfaceSubtypeMustBeCaseOfClosedType, WhenGuardNotSupported,
                 CaseClauseTypeNotSupported, OpenTypeNotSupported, MatchMustBeOnCaseType);
 
         public override void Initialize(AnalysisContext context)

ExhaustiveMatching.Analyzer/ExhaustiveMatching.Analyzer.csproj

@@ -3,9 +3,9 @@
   <PropertyGroup>
     <TargetFramework>netstandard2.0</TargetFramework>
     <IncludeBuildOutput>false</IncludeBuildOutput>
-    <AssemblyVersion>0.3.2.0</AssemblyVersion>
-    <FileVersion>0.3.2.0</FileVersion>
-    <Version>0.3.2</Version>
+    <AssemblyVersion>0.4.0.0</AssemblyVersion>
+    <FileVersion>0.4.0.0</FileVersion>
+    <Version>0.4.0</Version>
   </PropertyGroup>
 
   <ItemGroup>

ExhaustiveMatching.Analyzer/Resources.resx

@@ -180,6 +180,15 @@
   <data name="EM0014Title" xml:space="preserve">
     <value>Concrete Subtype of a Closed Type Must Be a Covered</value>
   </data>
+  <data name="EM0015Description" xml:space="preserve">
+    <value>An open interface subtype of a type marked with the "Closed" attribute must be a case of the closed type.</value>
+  </data>
+  <data name="EM0015Message" xml:space="preserve">
+    <value>Open interface {0} is not a case of its closed supertype: {1}</value>
+  </data>
+  <data name="EM0015Title" xml:space="preserve">
+    <value>Open Interface Subtype of a Closed Type Must Be a Case</value>
+  </data>
   <data name="EM0100Description" xml:space="preserve">
     <value>An exhustive switch statement does not support case clauses with "when" guards</value>
   </data>

ExhaustiveMatching.Analyzer/TypeDeclarationAnalyzer.cs

@@ -28,10 +28,13 @@ private static void MustBeCase(
             ITypeSymbol typeSymbol,
             INamedTypeSymbol closedAttribute)
         {
-            if (typeSymbol.IsAbstract)
+            var isConcrete = !typeSymbol.IsAbstract;
+            var isOpenInterface = typeSymbol.TypeKind == TypeKind.Interface
+                                  &&  !typeSymbol.HasAttribute(closedAttribute);
+            if (!isConcrete && !isOpenInterface)
                 return;
 
-            // Any concrete type directly inheriting from a closed type must be listed in the cases
+            // Any concrete type or open interface directly inheriting from a closed type must be listed in the cases
             var directSuperTypes = typeSymbol.DirectSuperTypes();
             var closedSuperTypes = directSuperTypes
                 .Where(t => t.HasAttribute(closedAttribute))
@@ -44,11 +47,18 @@ private static void MustBeCase(
                 if (isMember)
                     continue;
 
-                var diagnostic = Diagnostic.Create(ExhaustiveMatchAnalyzer.ConcreteSubtypeMustBeCaseOfClosedType,
-                    typeDeclaration.Identifier.GetLocation(), typeSymbol.GetFullName(), superType.GetFullName());
+                var descriptor = isConcrete
+                    ? ExhaustiveMatchAnalyzer.ConcreteSubtypeMustBeCaseOfClosedType
+                    // else isOpenInterface is always true
+                    : ExhaustiveMatchAnalyzer.OpenInterfaceSubtypeMustBeCaseOfClosedType;
+
+                var diagnostic = Diagnostic.Create(descriptor, typeDeclaration.Identifier.GetLocation(),
+                    typeSymbol.GetFullName(), superType.GetFullName());
                 context.ReportDiagnostic(diagnostic);
             }
 
+            if (!isConcrete) return;
+
             // Any concrete type indirectly inheriting from a closed type must be covered by a case type
             // that isn't itself closed. If it were closed, then it could be matched by all the cases, and
             // this type would not be matched.

ExhaustiveMatching/ExhaustiveMatching.csproj

@@ -2,9 +2,9 @@
 
   <PropertyGroup>
     <TargetFramework>netstandard2.0</TargetFramework>
-    <AssemblyVersion>0.3.2.0</AssemblyVersion>
-    <FileVersion>0.3.2.0</FileVersion>
-    <Version>0.3.2</Version>
+    <AssemblyVersion>0.4.0.0</AssemblyVersion>
+    <FileVersion>0.4.0.0</FileVersion>
+    <Version>0.4.0</Version>
   </PropertyGroup>
 
   <ItemGroup>

README.md

@@ -188,6 +188,10 @@ The analyzer reports various errors for incorrect code. The table below gives a
         <th>EM0014</th>
         <td>A concrete subtype of a closed type is not covered by some case</td>
     </tr>
+    <tr>
+        <th>EM0015</th>
+        <td>An open interface is not listed as a case in a closed type it is a direct subtype of</td>
+    </tr>
     <tr>
         <th>EM0100</th>
         <td>An exhaustive switch can't contain when guards</td>