From eeb9858080c0489956c4020bf1e00b1598051809 Mon Sep 17 00:00:00 2001
From: Andy Ayers <andya@microsoft.com>
Date: Wed, 18 Oct 2023 11:23:43 -0700
Subject: [PATCH] JIT: remove incorrect type deduction for an Unsafe.As case

Thanks to @SingleAccretion for the fix.

The JIT was assuming that an indirectly accessed value type field had
the type of the field, but that might not be the case if the field
was accessed via `Unsafe.As`.

Fix this by limiting type deduction from these indirectly accessed fields
to only ref type fields.

Closes #93650.
---
 src/coreclr/jit/gentree.cpp                   |  4 +-
 .../JitBlue/Runtime_93650/Runtime_93650.cs    | 42 +++++++++++++++++++
 .../Runtime_93650/Runtime_93650.csproj        |  8 ++++
 3 files changed, 53 insertions(+), 1 deletion(-)
 create mode 100644 src/tests/JIT/Regression/JitBlue/Runtime_93650/Runtime_93650.cs
 create mode 100644 src/tests/JIT/Regression/JitBlue/Runtime_93650/Runtime_93650.csproj

diff --git a/src/coreclr/jit/gentree.cpp b/src/coreclr/jit/gentree.cpp
index 122a8e35f1ee4..585ffa8680210 100644
--- a/src/coreclr/jit/gentree.cpp
+++ b/src/coreclr/jit/gentree.cpp
@@ -18752,10 +18752,12 @@ CORINFO_CLASS_HANDLE Compiler::gtGetFieldClassHandle(CORINFO_FIELD_HANDLE fieldH
             {
                 JITDUMP("Field's current class not available\n");
             }
+
+            return fieldClass;
         }
     }
 
-    return fieldClass;
+    return NO_CLASS_HANDLE;
 }
 
 //------------------------------------------------------------------------
diff --git a/src/tests/JIT/Regression/JitBlue/Runtime_93650/Runtime_93650.cs b/src/tests/JIT/Regression/JitBlue/Runtime_93650/Runtime_93650.cs
new file mode 100644
index 0000000000000..b424dd6c2f3c9
--- /dev/null
+++ b/src/tests/JIT/Regression/JitBlue/Runtime_93650/Runtime_93650.cs
@@ -0,0 +1,42 @@
+// 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.Runtime.CompilerServices;
+using System.Text;
+using Xunit;
+
+public struct Holder
+{
+    internal StringBuilder.AppendInterpolatedStringHandler _h;
+    public Holder() => _h = new(0, 0, new());
+
+    internal StringBuilder GetBuilder() => Unsafe.As<StringBuilder.AppendInterpolatedStringHandler, StringBuilder>(ref _h);
+}
+
+public static class Runtime_93650
+{
+    static int N = 1;
+
+    [Fact]
+    public static int Problem()
+    {
+        var sb = new Holder();
+        for (int i = 0; i < N; i++)
+        {
+            var s = Bind(ref sb);
+            if (s.Length != 0)
+            {
+                Console.WriteLine("FAILED: StringBuilder.ToString() returned: " + s);
+                return -1;
+            }
+        }
+
+        return 100;
+    }
+
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    public static string Bind(ref Holder parameters) => GetString(parameters.GetBuilder());
+
+    public static string GetString(StringBuilder sb) => sb.ToString();
+}
diff --git a/src/tests/JIT/Regression/JitBlue/Runtime_93650/Runtime_93650.csproj b/src/tests/JIT/Regression/JitBlue/Runtime_93650/Runtime_93650.csproj
new file mode 100644
index 0000000000000..15edd99711a1a
--- /dev/null
+++ b/src/tests/JIT/Regression/JitBlue/Runtime_93650/Runtime_93650.csproj
@@ -0,0 +1,8 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <Optimize>True</Optimize>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="$(MSBuildProjectName).cs" />
+  </ItemGroup>
+</Project>
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