Deduplicate base interfaces from another assembly in COM generator

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

@@ -53,7 +53,7 @@ public void Initialize(IncrementalGeneratorInitializationContext context)
             var externalInterfaceSymbols = attributedInterfaces.SelectMany(static (data, ct) =>
             {
                 return ComInterfaceInfo.CreateInterfaceInfoForBaseInterfacesInOtherCompilations(data.Symbol);
-            });
+            }).Collect().SelectMany(static (data, ct) => data.Distinct(ComInterfaceInfo.EqualityComparerForExternalIfaces.Instance));
 
             var interfaceSymbolsWithoutDiagnostics = interfaceSymbolsToGenerateWithoutDiagnostics.Concat(externalInterfaceSymbols);
 

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

@@ -11,6 +11,7 @@
 using InterfaceInfo = (Microsoft.Interop.ComInterfaceInfo InterfaceInfo, Microsoft.CodeAnalysis.INamedTypeSymbol Symbol);
 using DiagnosticOrInterfaceInfo = Microsoft.Interop.DiagnosticOr<(Microsoft.Interop.ComInterfaceInfo InterfaceInfo, Microsoft.CodeAnalysis.INamedTypeSymbol Symbol)>;
 using System.Diagnostics;
+using System.Collections.Generic;
 
 namespace Microsoft.Interop
 {
@@ -173,6 +174,13 @@ public static ImmutableArray<InterfaceInfo> CreateInterfaceInfoForBaseInterfaces
             return builder.ToImmutable();
         }
 
+        internal sealed class EqualityComparerForExternalIfaces : IEqualityComparer<(ComInterfaceInfo InterfaceInfo, INamedTypeSymbol Symbol)>
+        {
+            public bool Equals((ComInterfaceInfo, INamedTypeSymbol) x, (ComInterfaceInfo, INamedTypeSymbol) y) => SymbolEqualityComparer.Default.Equals(x.Item2, y.Item2);
+            public int GetHashCode((ComInterfaceInfo, INamedTypeSymbol) obj) => SymbolEqualityComparer.Default.GetHashCode(obj.Item2);
+            public static readonly EqualityComparerForExternalIfaces Instance = new();
+        }
+
         private static bool IsInPartialContext(INamedTypeSymbol symbol, InterfaceDeclarationSyntax syntax, [NotNullWhen(false)] out DiagnosticInfo? diagnostic)
         {
             // Verify that the types the interface is declared in are marked partial.

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

@@ -317,7 +317,7 @@ public partial interface IComInterface
                 using System.Runtime.CompilerServices;
                 using System.Runtime.InteropServices;
                 using System.Runtime.InteropServices.Marshalling;
-                
+
                 [GeneratedComInterface]
                 [Guid("0A617667-4961-4F90-B74F-6DC368E9817A")]
                 partial interface {|#1:IComInterface2|} : IComInterface
@@ -342,6 +342,55 @@ await VerifyInvocationWithMultipleProjectsAsync(
                 VerifyCS.DiagnosticWithArguments(GeneratorDiagnostics.BaseInterfaceDefinedInOtherAssembly, "IComInterface2", "IComInterface").WithLocation(1).WithSeverity(DiagnosticSeverity.Warning));
         }
 
+        [Fact]
+        public async Task ComInterfacesInheritingFromTheSameInterfaceAcrossCompilationsCalculatesCorrectVTableIndex()
+        {
+            string baseSource = $$"""
+                using System.Runtime.CompilerServices;
+                using System.Runtime.InteropServices;
+                using System.Runtime.InteropServices.Marshalling;
+
+                [GeneratedComInterface]
+                [Guid("0A617667-4961-4F90-B74F-6DC368E98179")]
+                public partial interface IComInterface
+                {
+                    void Method();
+                }
+                """;
+
+            string derivedSource = $$"""
+                using System.Runtime.CompilerServices;
+                using System.Runtime.InteropServices;
+                using System.Runtime.InteropServices.Marshalling;
+
+                [GeneratedComInterface]
+                [Guid("0A617667-4961-4F90-B74F-6DC368E9817A")]
+                partial interface {|#1:IComInterface2|} : IComInterface
+                {
+                    void DerivedMethod();
+                }
+
+                [GeneratedComInterface]
+                [Guid("0A617667-4961-4F93-B74F-6DC368E9817A")]
+                partial interface {|#2:IComInterface3|} : IComInterface
+                {
+                    void DerivedMethod();
+                }
+                """;
+
+            await VerifyInvocationWithMultipleProjectsAsync(
+                derivedSource,
+                baseSource,
+                "IComInterface2",
+                "DerivedMethod",
+                (newComp, invocation) =>
+                {
+                    Assert.Equal(4, Assert.IsAssignableFrom<ILiteralOperation>(Assert.IsAssignableFrom<IConversionOperation>(invocation.Target).Operand.ChildOperations.Last()).ConstantValue.Value);
+                },
+                VerifyCS.DiagnosticWithArguments(GeneratorDiagnostics.BaseInterfaceDefinedInOtherAssembly, "IComInterface2", "IComInterface").WithLocation(1).WithSeverity(DiagnosticSeverity.Warning),
+                VerifyCS.DiagnosticWithArguments(GeneratorDiagnostics.BaseInterfaceDefinedInOtherAssembly, "IComInterface3", "IComInterface").WithLocation(2).WithSeverity(DiagnosticSeverity.Warning));
+        }
+
         [Fact]
         public async Task ComInterfaceInheritingAcrossCompilationsChainInBaseCalculatesCorrectVTableIndex()
         {
@@ -369,7 +418,7 @@ public partial interface IComInterface2 : IComInterface
                 using System.Runtime.CompilerServices;
                 using System.Runtime.InteropServices;
                 using System.Runtime.InteropServices.Marshalling;
-                
+
                 [GeneratedComInterface]
                 [Guid("0A617667-4961-4F90-B74F-6DC368E9817A")]
                 partial interface {|#1:IComInterface3|} : IComInterface2