Import isinst/castclass Nullable as underlying type

src/coreclr/jit/importer.cpp

@@ -5426,6 +5426,23 @@ GenTree* Compiler::impCastClassOrIsInstToTree(
     bool shouldExpandInline = true;
     bool isClassExact       = impIsClassExact(pResolvedToken->hClass);
 
+    // ECMA-335 III.4.3:  If typeTok is a nullable type, Nullable<T>, it is interpreted as "boxed" T
+    // We can convert constant-ish tokens of nullable to its underlying type.
+    // However, when the type is shared generic parameter like Nullable<Struct<__Canon>>, the actual type will require
+    // runtime lookup. It's too complex to add another level of indirection in op2, fallback to the cast helper instead.
+    if (isClassExact && !(info.compCompHnd->getClassAttribs(pResolvedToken->hClass) & CORINFO_FLG_SHAREDINST))
+    {
+        CORINFO_CLASS_HANDLE hClass = info.compCompHnd->getTypeForBox(pResolvedToken->hClass);
+
+        if (hClass != pResolvedToken->hClass)
+        {
+            bool runtimeLookup;
+            pResolvedToken->hClass = hClass;
+            op2                    = impTokenToHandle(pResolvedToken, &runtimeLookup);
+            assert(!runtimeLookup);
+        }
+    }
+
     // Profitability check.
     //
     // Don't bother with inline expansion when jit is trying to generate code quickly

src/tests/JIT/opt/Casts/castclass_valuetype.il

@@ -0,0 +1,60 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+
+.assembly extern mscorlib { auto }
+.assembly extern xunit.core {}
+.assembly extern System.Console { auto }
+.assembly extern System.Runtime { .publickeytoken = (B0 3F 5F 7F 11 D5 0A 3A ) }
+
+.assembly 'castclass_valuetype' { }
+
+.class public auto Program extends [System.Runtime]System.Object
+{
+  .method public hidebysig static int32 Main() cil managed
+  {
+    .custom instance void [xunit.core]Xunit.FactAttribute::.ctor() = (
+        01 00 00 00
+    )
+    .entrypoint
+    .maxstack 8
+
+    .try
+    {
+      // castclass valuetype
+      ldc.i4 1234
+      box int32
+      castclass int32
+      unbox int32
+      call instance string [System.Runtime]System.Int32::ToString()
+      call void [System.Console]System.Console::WriteLine(class [System.Runtime]System.String)
+
+      // castclass nullable
+      ldc.i4 1234
+      box int32
+      castclass valuetype [System.Runtime]System.Nullable`1<int32>
+      unbox int32
+      call instance string [System.Runtime]System.Int32::ToString()
+      call void [System.Console]System.Console::WriteLine(class [System.Runtime]System.String)
+
+      leave.s SUCCESS
+    }
+    catch [System.Runtime]System.Object
+    {
+      pop
+      leave.s FAIL
+    }
+
+SUCCESS:
+    ldstr "SUCCESS"
+    call void [System.Console]System.Console::WriteLine(class [System.Runtime]System.String)
+    ldc.i4 100
+    ret
+
+FAIL:
+    ldstr "FAILED"
+    call void [System.Console]System.Console::WriteLine(class [System.Runtime]System.String)
+    ldc.i4 0
+    ret
+  }
+}

src/tests/JIT/opt/Casts/castclass_valuetype.ilproj

@@ -0,0 +1,11 @@
+<Project Sdk="Microsoft.NET.Sdk.IL">
+  <PropertyGroup>
+    <CLRTestPriority>1</CLRTestPriority>
+  </PropertyGroup>
+  <PropertyGroup>
+    <DebugType>PdbOnly</DebugType>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="castclass_valuetype.il" />
+  </ItemGroup>
+</Project>