Remove forced override of package contents

src/Microsoft.CodeAnalysis.Testing/Microsoft.CodeAnalysis.Analyzer.Testing/ReferenceAssemblies.cs

@@ -460,14 +460,58 @@ await PackageExtractor.ExtractPackageAsync(
                     }
                 }
 
-                // Prefer assemblies from the reference assembly package to ones otherwise provided
+                // Prefer newer assemblies when more than one have the same name
                 if (ReferenceAssemblyPackage is not null)
                 {
-                    var referenceAssemblies = new HashSet<string>(resolvedAssemblies.Where(resolved => resolved.StartsWith(referenceAssemblyInstalledPath!)));
+                    var comparer = new FrameworkPrecedenceSorter(DefaultFrameworkNameProvider.Instance, allEquivalent: false);
+                    var assembliesByName = resolvedAssemblies.GroupBy(Path.GetFileNameWithoutExtension, StringComparer.OrdinalIgnoreCase);
 
-                    // Suppression due to https://github.com/dotnet/roslyn/issues/44735
-                    var referenceAssemblyNames = new HashSet<string>(referenceAssemblies.Select((Func<string, string>)Path.GetFileNameWithoutExtension!));
-                    resolvedAssemblies.RemoveWhere(resolved => referenceAssemblyNames.Contains(Path.GetFileNameWithoutExtension(resolved)) && !referenceAssemblies.Contains(resolved));
+                    // Keep track of assemblies to remove from resolvedAssemblies. Defer the actual removal to the end
+                    // of this block for ease in future debugging scenarios.
+                    var assembliesToRemove = new List<string>();
+                    foreach (var assemblyNameGroup in assembliesByName)
+                    {
+                        var assembliesByPrecedence = assemblyNameGroup
+                            .Select(static name => (name, framework: GetFrameworkNameFromPath(name)))
+                            .OrderBy(static x => x.framework, comparer)
+                            .ThenByDescending(static x => x.framework, new NuGetFrameworkSorter())
+                            .ToArray();
+                        for (var i = 1; i < assembliesByPrecedence.Length; i++)
+                        {
+                            // We want to keep the last reference listed for the most recent supported target framework.
+                            // Typically, if more than one item has the most recent supported target framework, it will
+                            // be a case where the reference assembly package provides the assembly and a newer version
+                            // is provided explicitly. For example:
+                            //
+                            // Microsoft.NETCore.App.Ref 6.0.0 provides System.Collections.Immutable in the net6.0 folder
+                            // System.Collections.Immutable 8.0.0 provides System.Collections.Immutable in the net6.0 folder
+                            //
+                            // In this example, the Microsoft.NETCore.App.Ref package is resolved first, so by taking
+                            // the last net6.0 assembly, we ensure the assembly from System.Collections.Immutable 8.0.0
+                            // is resolved.
+                            if (Equals(assembliesByPrecedence[0].framework, assembliesByPrecedence[i].framework))
+                            {
+                                assembliesToRemove.Add(assembliesByPrecedence[i - 1].name);
+                            }
+                            else
+                            {
+                                assembliesToRemove.Add(assembliesByPrecedence[i].name);
+                            }
+                        }
+
+                        static NuGetFramework GetFrameworkNameFromPath(string path)
+                        {
+                            var frameworkFolder = Path.GetFileName(Path.GetDirectoryName(path));
+                            if (frameworkFolder is null)
+                            {
+                                return NuGetFramework.UnsupportedFramework;
+                            }
+
+                            return NuGetFramework.ParseFolder(frameworkFolder);
+                        }
+                    }
+
+                    resolvedAssemblies.ExceptWith(assembliesToRemove);
                 }
 
                 // Add the facade assemblies

tests/Microsoft.CodeAnalysis.Testing/Microsoft.CodeAnalysis.Analyzer.Testing.UnitTests/MetadataReferenceTests.cs

@@ -3,6 +3,7 @@
 // See the LICENSE file in the project root for more information.
 
 using System;
+using System.Collections.Immutable;
 using System.Threading;
 using System.Threading.Tasks;
 using Xunit;
@@ -649,6 +650,37 @@ class TestClass {
             }.RunAsync();
         }
 
+        [Theory]
+        [InlineData("net462")]
+        [InlineData("net47")]
+        [InlineData("net471")]
+        [InlineData("net472")]
+        [InlineData("net48")]
+#if !(NETCOREAPP1_1 || NET46)
+        [InlineData("net6.0")]
+        [InlineData("net7.0")]
+        [InlineData("net8.0")]
+#endif
+        [InlineData("netstandard2.0")]
+        [InlineData("netstandard2.1")]
+        public async Task ResolveSystemCollectionsImmutable8(string targetFramework)
+        {
+            var testCode = @"
+using System.Collections.Frozen;
+
+class TestClass {
+  FrozenSet<int> TestMethod() => throw null;
+}
+";
+
+            await new CSharpTest()
+            {
+                TestCode = testCode,
+                ReferenceAssemblies = ReferenceAssembliesForTargetFramework(targetFramework)
+                    .AddPackages(ImmutableArray.Create(new PackageIdentity("System.Collections.Immutable", "8.0.0"))),
+            }.RunAsync();
+        }
+
         internal static ReferenceAssemblies ReferenceAssembliesForTargetFramework(string targetFramework)
         {
             return targetFramework switch