From ce660afb6e96b6923251ce8ab8ca38814d1b44a6 Mon Sep 17 00:00:00 2001
From: Ryan Shepherd <ryansh@microsoft.com>
Date: Wed, 3 Jun 2026 20:33:12 -0700
Subject: [PATCH] Stop exporting winrt::impl::get_marshaler to workaround MSVC
 modules bug

---
 strings/base_marshaler.h                      |  2 +-
 test/test_cpp20_module/marshal.cpp            | 24 +++++++++++++++++++
 .../test_cpp20_module.vcxproj                 |  4 +++-
 3 files changed, 28 insertions(+), 2 deletions(-)
 create mode 100644 test/test_cpp20_module/marshal.cpp

diff --git a/strings/base_marshaler.h b/strings/base_marshaler.h
index 9be6959c9..526f4d4c3 100644
--- a/strings/base_marshaler.h
+++ b/strings/base_marshaler.h
@@ -1,5 +1,5 @@
 
-WINRT_EXPORT namespace winrt::impl
+namespace winrt::impl
 {
     inline std::int32_t make_marshaler(unknown_abi* outer, void** result) noexcept
     {
diff --git a/test/test_cpp20_module/marshal.cpp b/test/test_cpp20_module/marshal.cpp
new file mode 100644
index 000000000..8024b2413
--- /dev/null
+++ b/test/test_cpp20_module/marshal.cpp
@@ -0,0 +1,24 @@
+#include "pch.h"
+#include <objidl.h>
+
+import std;
+import winrt.Windows.Foundation.Collections;
+
+using namespace winrt;
+
+struct S : implements<S, Windows::Foundation::IStringable>
+{
+    hstring ToString()
+    {
+        return L"S";
+    }
+};
+
+// When winrt::impl::get_marshaler was being exported, an MSVC bug caused the marshaler
+// object to have a null vtable, which caused a crash when calling any method on the marshaler.
+// This test ensures that the marshaler vtable is properly initialized.
+TEST_CASE("IMarshal")
+{
+    auto s = make<S>();
+    auto marshal = s.as<IMarshal>();
+}
diff --git a/test/test_cpp20_module/test_cpp20_module.vcxproj b/test/test_cpp20_module/test_cpp20_module.vcxproj
index 1e76c487b..f5c20a35d 100644
--- a/test/test_cpp20_module/test_cpp20_module.vcxproj
+++ b/test/test_cpp20_module/test_cpp20_module.vcxproj
@@ -25,7 +25,8 @@
       <Configuration>Release</Configuration>
       <Platform>x64</Platform>
     </ProjectConfiguration>
-  </ItemGroup>  <PropertyGroup Label="Globals">
+  </ItemGroup>
+  <PropertyGroup Label="Globals">
     <VCProjectVersion>16.0</VCProjectVersion>
     <ProjectGuid>{B8E3A5CE-4E91-4F27-9B02-E0CAF7E10D72}</ProjectGuid>
     <RootNamespace>test_cpp20_module</RootNamespace>
@@ -91,6 +92,7 @@
     <ClInclude Include="pch.h" />
   </ItemGroup>
   <ItemGroup>
+    <ClCompile Include="marshal.cpp" />
     <ClCompile Include="pch.cpp">
       <PrecompiledHeader>Create</PrecompiledHeader>
     </ClCompile>