Fix missing duplicate diagnostic for enum declarations

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

@@ -847,18 +847,18 @@ internal override ManagedKind GetManagedKind(ref CompoundUseSiteInfo<AssemblySym
 
         internal bool IsUnsafe => HasFlag(DeclarationModifiers.Unsafe);
 
-        internal SyntaxTree AssociatedSyntaxTree => declaration.Declarations[0].Location.SourceTree;
+        private SyntaxTree? AssociatedSyntaxTree => IsFileLocal ? declaration.Declarations[0].Location.SourceTree : null;
 
         internal sealed override FileIdentifier? AssociatedFileIdentifier
         {
             get
             {
-                if (!IsFileLocal)
+                if (AssociatedSyntaxTree is not SyntaxTree syntaxTree)
                 {
                     return null;
                 }
 
-                return FileIdentifier.Create(AssociatedSyntaxTree);
+                return FileIdentifier.Create(syntaxTree);
             }
         }
 
@@ -1309,6 +1309,11 @@ private Dictionary<ReadOnlyMemory<char>, ImmutableArray<NamedTypeSymbol>> MakeTy
             var conflictDict = new Dictionary<(string name, int arity, SyntaxTree? syntaxTree), SourceNamedTypeSymbol>();
             try
             {
+                // Declarations which can be merged into a single type symbol have already been merged at this phase.
+                // Merging behaves the same in either presence or absence of 'partial' modifiers.
+                // However, type declarations which can never be partial won't merge, e.g. 'enum',
+                // and type declarations with different kinds, e.g. 'class' and 'struct' will never merge.
+                // Now we want to figure out if declarations which didn't merge have name conflicts.
                 foreach (var childDeclaration in declaration.Children)
                 {
                     var t = new SourceNamedTypeSymbol(this, childDeclaration, diagnostics);

src/Compilers/CSharp/Test/Symbol/Symbols/Source/FileModifierTests.cs

@@ -448,6 +448,69 @@ void symbolValidator(ModuleSymbol symbol)
         }
     }
 
+    [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/67258")]
+    public void NonFileLocalClass_Duplicate()
+    {
+        var source = @"
+public class D { }
+
+public partial class C
+{
+    public class D { }
+}
+";
+        var comp = CreateCompilation(new[] { source, source });
+        comp.VerifyEmitDiagnostics(
+            // 1.cs(2,14): error CS0101: The namespace '<global namespace>' already contains a definition for 'D'
+            // public class D { }
+            Diagnostic(ErrorCode.ERR_DuplicateNameInNS, "D").WithArguments("D", "<global namespace>").WithLocation(2, 14),
+            // 1.cs(6,18): error CS0102: The type 'C' already contains a definition for 'D'
+            //     public class D { }
+            Diagnostic(ErrorCode.ERR_DuplicateNameInClass, "D").WithArguments("C", "D").WithLocation(6, 18));
+    }
+
+    [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/67258")]
+    public void NonFileLocalEnum_Duplicate()
+    {
+        var source = @"
+public enum E { }
+
+public partial class C
+{
+    public enum E { }
+}
+";
+        var comp = CreateCompilation(new[] { source, source });
+        comp.VerifyEmitDiagnostics(
+            // 1.cs(2,13): error CS0101: The namespace '<global namespace>' already contains a definition for 'E'
+            // public enum E { }
+            Diagnostic(ErrorCode.ERR_DuplicateNameInNS, "E").WithArguments("E", "<global namespace>").WithLocation(2, 13),
+            // 1.cs(6,17): error CS0102: The type 'C' already contains a definition for 'E'
+            //     public enum E { }
+            Diagnostic(ErrorCode.ERR_DuplicateNameInClass, "E").WithArguments("C", "E").WithLocation(6, 17));
+    }
+
+    [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/67258")]
+    public void NonFileLocalDelegate_Duplicate()
+    {
+        var source = @"
+public delegate void D();
+
+public partial class C
+{
+    public delegate void D();
+}
+";
+        var comp = CreateCompilation(new[] { source, source });
+        comp.VerifyEmitDiagnostics(
+            // 1.cs(2,22): error CS0101: The namespace '<global namespace>' already contains a definition for 'D'
+            // public delegate void D();
+            Diagnostic(ErrorCode.ERR_DuplicateNameInNS, "D").WithArguments("D", "<global namespace>").WithLocation(2, 22),
+            // 1.cs(6,26): error CS0102: The type 'C' already contains a definition for 'D'
+            //     public delegate void D();
+            Diagnostic(ErrorCode.ERR_DuplicateNameInClass, "D").WithArguments("C", "D").WithLocation(6, 26));
+    }
+
     [Fact]
     public void OtherFileUse()
     {