Strip partial and virtual modifiers from generated COM interface method stubs

src/libraries/System.Runtime.InteropServices/gen/ComInterfaceGenerator/ComInterfaceGenerator.cs

@@ -363,7 +363,7 @@ internal static IncrementalMethodStubGenerationContext CalculateStubInformation(
 
             var containingSyntaxContext = new ContainingSyntaxContext(syntax);
             var methodSyntaxTemplate = new ContainingSyntax(
-                new SyntaxTokenList(syntax.Modifiers.Where(static m => !m.IsKind(SyntaxKind.NewKeyword))).StripAccessibilityModifiers(),
+                new SyntaxTokenList(syntax.Modifiers.Where(static m => !m.IsKind(SyntaxKind.NewKeyword) && !m.IsKind(SyntaxKind.PartialKeyword))).StripAccessibilityModifiers(),
                 SyntaxKind.MethodDeclaration,
                 syntax.Identifier,
                 syntax.TypeParameterList);

src/libraries/System.Runtime.InteropServices/gen/ComInterfaceGenerator/VtableIndexStubGenerator.cs

@@ -268,7 +268,7 @@ internal static SourceAvailableIncrementalMethodStubGenerationContext CalculateS
 
             var containingSyntaxContext = new ContainingSyntaxContext(syntax);
 
-            var methodSyntaxTemplate = new ContainingSyntax(syntax.Modifiers.StripAccessibilityModifiers(), SyntaxKind.MethodDeclaration, syntax.Identifier, syntax.TypeParameterList);
+            var methodSyntaxTemplate = new ContainingSyntax(new SyntaxTokenList(syntax.Modifiers.Where(static m => !m.IsKind(SyntaxKind.PartialKeyword))).StripAccessibilityModifiers(), SyntaxKind.MethodDeclaration, syntax.Identifier, syntax.TypeParameterList);
 
             ImmutableArray<FunctionPointerUnmanagedCallingConventionSyntax> callConv = VirtualMethodPointerStubGenerator.GenerateCallConvSyntaxFromAttributes(suppressGCTransitionAttribute, unmanagedCallConvAttribute, defaultCallingConventions: ImmutableArray<FunctionPointerUnmanagedCallingConventionSyntax>.Empty);
 

src/libraries/System.Runtime.InteropServices/tests/ComInterfaceGenerator.Unit.Tests/Compiles.cs

@@ -376,6 +376,36 @@ public async Task ValidateComInterfaceSnippets(string id, string source)
             await VerifyComInterfaceGenerator.VerifySourceGeneratorAsync(source);
         }
 
+        [Fact]
+        public async Task PartialMethodModifierOnComInterfaceMethodCompiles()
+        {
+            string source = """
+                using System.Runtime.InteropServices;
+                using System.Runtime.InteropServices.Marshalling;
+
+                [GeneratedComInterface]
+                [Guid("9D3FD745-3C90-4C10-B140-FAFB01E3541D")]
+                internal partial interface IComInterface
+                {
+                    void Method();
+                    public partial void PartialMethod();
+                }
+                internal partial interface IComInterface
+                {
+                    public partial void PartialMethod() { }
+                }
+                """;
+
+            // CS0539 is expected because the partial method's default implementation
+            // conflicts with the generated explicit interface implementation.
+            // The important verification is that no additional errors are produced
+            // from the 'partial' modifier being incorrectly copied to generated code.
+            await VerifyComInterfaceGenerator.VerifySourceGeneratorAsync(source,
+                DiagnosticResult.CompilerError("CS0539")
+                    .WithSpan("Microsoft.Interop.ComInterfaceGenerator/Microsoft.Interop.ComInterfaceGenerator/IComInterface.cs", 78, 32, 78, 45)
+                    .WithArguments("InterfaceImplementation.PartialMethod()"));
+        }
+
         [Fact]
         public async Task DocumentedComInterfaceDoesNotProduceCS1591Warnings()
         {