Fix stack overflow when partial event implements interface

src/Compilers/CSharp/Portable/Symbols/Source/SourceMemberContainerSymbol.cs

@@ -3976,12 +3976,25 @@ private static void MergePartialMembers(
             {
                 membersBySignature.Clear();
 
+                // Track if this name has any accessors to avoid false diagnostics for methods with the same name
+                bool hasAccessors = false;
+
                 if (pair.Value is ArrayBuilder<Symbol> arrayBuilder)
                 {
                     foreach (var symbol in arrayBuilder)
                     {
                         Debug.Assert(symbol.IsPartialMember());
 
+                        // Skip accessors from signature comparison - they are handled by their associated member.
+                        // Comparing accessor signatures would trigger computation of parameter types,
+                        // which for event accessors requires checking IsWindowsRuntimeEvent, potentially causing
+                        // infinite recursion during member initialization.
+                        if (symbol is SourcePropertyAccessorSymbol or SourceEventAccessorSymbol)
+                        {
+                            hasAccessors = true;
+                            continue;
+                        }
+
                         if (!membersBySignature.TryGetValue(symbol, out var prev))
                         {
                             membersBySignature.Add(symbol, symbol);
@@ -4024,7 +4037,15 @@ private static void MergePartialMembers(
                 {
                     var symbol = (Symbol)pair.Value;
                     Debug.Assert(symbol.IsPartialMember());
-                    membersBySignature.Add(symbol, symbol);
+
+                    if (symbol is SourcePropertyAccessorSymbol or SourceEventAccessorSymbol)
+                    {
+                        hasAccessors = true;
+                    }
+                    else
+                    {
+                        membersBySignature.Add(symbol, symbol);
+                    }
                 }
 
                 foreach (var symbol in membersBySignature.Values)
@@ -4033,7 +4054,8 @@ private static void MergePartialMembers(
                     {
                         case SourceOrdinaryMethodSymbol method:
                             // partial implementations not paired with a definition
-                            if (method.IsPartialImplementation && method.OtherPartOfPartial is null)
+                            // Skip diagnostic if there are accessors with the same name (e.g., partial method get_P() coexisting with property P's accessor)
+                            if (!hasAccessors && method.IsPartialImplementation && method.OtherPartOfPartial is null)
                             {
                                 diagnostics.Add(ErrorCode.ERR_PartialMethodMustHaveLatent, method.GetFirstLocation(), method);
                             }

src/Compilers/CSharp/Test/Emit3/PartialEventsAndConstructorsTests.cs

@@ -3740,4 +3740,29 @@ [Obsolete] void M() { }
             //     [method: Obsolete] public partial event Action H;
             Diagnostic(ErrorCode.ERR_AttributeNotOnEventAccessor, "Obsolete").WithArguments("System.ObsoleteAttribute", "class, struct, enum, constructor, method, property, indexer, field, event, interface, delegate").WithLocation(35, 14));
     }
+
+    [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/76879")]
+    public void PartialEventImplementingInterface_NoStackOverflow()
+    {
+        var source = """
+            using System;
+
+            internal interface IC
+            {
+                event EventHandler E;
+            }
+
+            partial class C : IC
+            {
+                public partial event EventHandler E;
+            }
+
+            partial class C
+            {
+                public partial event EventHandler E { add { } remove { } }
+            }
+            """;
+        var comp = CreateCompilation(source);
+        comp.VerifyDiagnostics();
+    }
 }