Add MSGH006 diagnostic for SwitchCase argument type mismatch (#41)

* Initial plan

* Add MSGH006 diagnostic for SwitchCase argument type mismatch

Co-authored-by: dex3r <3155725+dex3r@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: dex3r <3155725+dex3r@users.noreply.github.com>

Author: Copilot

MattSourceGenHelpers.GeneratorTests/GeneratorDiagnosticsTests.cs

@@ -244,6 +244,62 @@ private static int GetValue_Generator(int key)
             "Error message should hint that a partial method was called during generation");
     }
 
+    // -----------------------------------------------------------------------
+    // MSGH006 – SwitchCase argument type mismatch
+    // -----------------------------------------------------------------------
+
+    [Test]
+    public void GeneratesMethod_SwitchCaseWithWrongArgumentType_EmitsMSGH006()
+    {
+        string source = """
+            using MattSourceGenHelpers.Abstractions;
+
+            namespace TestNamespace;
+
+            public static partial class MyClass
+            {
+                public static partial int GetValue(int key);
+
+                [GeneratesMethod(nameof(GetValue))]
+                [SwitchCase(arg1: "aaa")]
+                private static int GetValue_Generator(int key) => 42;
+            }
+            """;
+
+        ImmutableArray<Diagnostic> diagnostics = GeneratorTestHelper.GetGeneratorOnlyDiagnostics(source);
+
+        Diagnostic? msgh006 = diagnostics.FirstOrDefault(d => d.Id == "MSGH006");
+        Assert.That(msgh006, Is.Not.Null, "Expected MSGH006 diagnostic for SwitchCase argument type mismatch");
+        Assert.That(msgh006!.GetMessage(), Does.Contain("string"),
+            "Error message should mention the provided type");
+        Assert.That(msgh006.GetMessage(), Does.Contain("int"),
+            "Error message should mention the expected parameter type");
+    }
+
+    [Test]
+    public void GeneratesMethod_SwitchCaseWithCorrectArgumentType_DoesNotEmitMSGH006()
+    {
+        string source = """
+            using MattSourceGenHelpers.Abstractions;
+
+            namespace TestNamespace;
+
+            public static partial class MyClass
+            {
+                public static partial int GetValue(int key);
+
+                [GeneratesMethod(nameof(GetValue))]
+                [SwitchCase(arg1: 1)]
+                private static int GetValue_Generator(int key) => 42;
+            }
+            """;
+
+        ImmutableArray<Diagnostic> diagnostics = GeneratorTestHelper.GetGeneratorOnlyDiagnostics(source);
+
+        Assert.That(diagnostics.Any(d => d.Id == "MSGH006"), Is.False,
+            "Should not emit MSGH006 when SwitchCase argument type matches the parameter type");
+    }
+
     // -----------------------------------------------------------------------
     // Type mismatch between generator return type and partial method return type
     // -----------------------------------------------------------------------

MattSourceGenHelpers.Generators/AnalyzerReleases.Unshipped.md

@@ -6,4 +6,5 @@ MSGH001 | GeneratesMethodGenerator | Error | GeneratesMethodGenerator
 MSGH002 | GeneratesMethodGenerator | Error | GeneratesMethodGenerator
 MSGH003 | GeneratesMethodGenerator | Disabled | GeneratesMethodGenerator
 MSGH004 | GeneratesMethodGenerator | Error | GeneratesMethodGenerator
-MSGH005 | GeneratesMethodGenerator | Error | GeneratesMethodGenerator
+MSGH005 | GeneratesMethodGenerator | Error | GeneratesMethodGenerator
+MSGH006 | GeneratesMethodGenerator | Error | GeneratesMethodGenerator

MattSourceGenHelpers.Generators/GeneratesMethodGenerator.Diagnostics.cs

@@ -45,4 +45,12 @@ internal static class GeneratesMethodGeneratorDiagnostics
         category: Category,
         defaultSeverity: DiagnosticSeverity.Error,
         isEnabledByDefault: true);
+
+    internal static readonly DiagnosticDescriptor SwitchCaseArgumentTypeMismatchError = new(
+        id: "MSGH006",
+        title: "SwitchCase argument type mismatch",
+        messageFormat: "SwitchCase argument type '{0}' does not match the method parameter type '{1}'",
+        category: Category,
+        defaultSeverity: DiagnosticSeverity.Error,
+        isEnabledByDefault: true);
 }

MattSourceGenHelpers.Generators/GeneratesMethodPatternSourceBuilder.cs

@@ -69,6 +69,24 @@ internal static string GenerateFromSwitchAttributes(
                     continue;
                 }
 
+                if (partialMethod.Parameters.Length > 0)
+                {
+                    ITypeSymbol? switchArgType = switchCaseAttribute.ConstructorArguments[switchCaseArgIndex].Type;
+                    ITypeSymbol partialMethodParamType = partialMethod.Parameters[0].Type;
+
+                    if (switchArgType != null && !SymbolEqualityComparer.Default.Equals(switchArgType, partialMethodParamType))
+                    {
+                        Location attributeLocation = switchCaseAttribute.ApplicationSyntaxReference?.GetSyntax()?.GetLocation()
+                            ?? switchMethod.Syntax.GetLocation();
+                        context.ReportDiagnostic(Diagnostic.Create(
+                            GeneratesMethodGeneratorDiagnostics.SwitchCaseArgumentTypeMismatchError,
+                            attributeLocation,
+                            switchArgType.ToDisplayString(),
+                            partialMethodParamType.ToDisplayString()));
+                        continue;
+                    }
+                }
+
                 (string? result, string? error) = GeneratesMethodExecutionRuntime.ExecuteGeneratorMethodWithArgs(
                     switchMethod.Symbol,
                     allPartials,