Fix issue with array interface devirtualization

src/coreclr/vm/jitinterface.cpp

@@ -8604,18 +8604,18 @@ bool CEEInfo::resolveVirtualMethodHelper(CORINFO_DEVIRTUALIZATION_INFO * info)
 
         if (pObjMT->IsArray())
         {
+            if (pBaseMT->IsSharedByGenericInstantiations())
+            {
+                info->detail = CORINFO_DEVIRTUALIZATION_FAILED_CANON;
+                return false;
+            }
+
             // Does the array implicitly implement this interface?
             //
             isArrayImplicitInterface = pBaseMT->HasInstantiation() && IsImplicitInterfaceOfSZArray(pBaseMT);
 
             if (!isArrayImplicitInterface)
             {
-                if (pBaseMT->IsSharedByGenericInstantiations())
-                {
-                    info->detail = CORINFO_DEVIRTUALIZATION_FAILED_CANON;
-                    return false;
-                }
-
                 // Ensure we can cast the array to the interface type
                 //
                 if (!TypeHandle(pObjMT).CanCastTo(TypeHandle(pBaseMT)))
@@ -8641,15 +8641,12 @@ bool CEEInfo::resolveVirtualMethodHelper(CORINFO_DEVIRTUALIZATION_INFO * info)
             {
                 _ASSERTE(pObjMT->IsArray());
 
-                // We cannot devirtualize unless we know the exact array element type
-                //
-                TypeHandle elemType = pObjMT->GetArrayElementTypeHandle();
-                if (elemType.IsCanonicalSubtype())
-                {
-                    info->detail = CORINFO_DEVIRTUALIZATION_FAILED_LOOKUP;
-                    return false;
-                }
-                pDevirtMD = GetActualImplementationForArrayGenericIListOrIReadOnlyListMethod(pBaseMD, elemType);
+                // The instantiation we want is based on the interface element type, not the
+                // array element type.
+                TypeHandle resultElemType = pBaseMT->GetInstantiation()[0];
+                // We should have ruled this out above.
+                _ASSERTE(!resultElemType.IsCanonicalSubtype());
+                pDevirtMD = GetActualImplementationForArrayGenericIListOrIReadOnlyListMethod(pBaseMD, resultElemType);
             }
             else if (pObjMT->IsSharedByGenericInstantiations() || pBaseMT->IsSharedByGenericInstantiations())
             {

src/tests/JIT/Regression/JitBlue/Runtime_120270/Runtime_120270.cs

@@ -0,0 +1,95 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Runtime.CompilerServices;
+using Xunit;
+
+class Base{}
+class Derived : Base {}
+
+public class Runtime_120270
+{
+    [Fact]
+    public static int TestEntryPoint()
+    {
+        int i = 0;
+        bool failed = false;
+        try
+        {
+            while (i < 100_000)
+            {
+                i++;
+                var values = new[] { DayOfWeek.Saturday }.Cast<int>();
+                foreach (var value in values)
+                {
+                }
+            }
+        }
+        catch (Exception ex)
+        {
+            Console.WriteLine(i);
+            Console.WriteLine(ex);
+            failed = true;
+        }
+
+        return failed ? -1 : 100;
+    }
+
+    [Fact]
+    public static int TestEntryPoint2()
+    {
+        bool failed = false;
+        try
+        {
+            Foo<Base>([new Derived()]);
+        }
+        catch (Exception ex)
+        {
+            Console.WriteLine(ex);
+            failed = true;
+        }
+        return failed ? -1 : 100;
+    }
+
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    static void Foo<T>(object[] x)
+    {
+        int i = 0;
+        while (i < 100_000)
+        {
+            i++;
+            var values = x.Cast<T>();
+            foreach (var value in values)
+            {
+            }
+        }
+    }
+
+    [Fact]
+    public static int TestEntryPoint3()
+    {
+        for (int i = 0; i < 100_000; i++)
+        {
+            Derived[] d = [new Derived()];
+            IEnumerable<Base> e = d;
+            IEnumerator<Base> en = e.GetEnumerator();
+            string s = en.GetType().ToString();
+
+            if (i == 0)
+            {
+                Console.WriteLine($"Enumerator type: {s}");
+            }
+
+            if (!s.Equals("System.SZGenericArrayEnumerator`1[Base]"))
+            {
+                Console.WriteLine($"Enumerator type changed at {i} to {s}");
+                return -1;
+            }
+        }
+        return 100;
+    }
+}
+

src/tests/JIT/Regression/JitBlue/Runtime_120270/Runtime_120270.csproj

@@ -0,0 +1,9 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <DebugType>None</DebugType>
+    <Optimize>True</Optimize>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="$(MSBuildProjectName).cs" />
+  </ItemGroup>
+</Project>