Fix issue with array interface devirtualization

src/coreclr/vm/jitinterface.cpp

@@ -8648,15 +8648,11 @@ 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];
+                _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 +1,36 @@
+// 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.Linq;
+using Xunit;
+
+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;
+    }
+}
+

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>