Suppress unexpected compiler warnings

proxy.h

@@ -656,11 +656,40 @@ struct proxy_helper {
       return static_cast<add_qualifier_t<proxy<F>, Q>>(
           std::forward<add_qualifier_t<A, Q>>(a));
     } else {
+      // Note: The use of offsetof below is technically undefined until C++20
+      // because proxy may not be a standard layout type. However, all compilers
+      // currently provide well-defined behavior as an extension (which is
+      // demonstrated since constexpr evaluation must diagnose all undefined
+      // behavior). However, various compilers also warn about this use of
+      // offsetof, which must be suppressed.
+#if defined(__INTEL_COMPILER)
+#pragma warning push
+#pragma warning(disable : 1875)
+#elif defined(__GNUC__) || defined(__clang__)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Winvalid-offsetof"
+#elif defined(__NVCC__)
+#pragma nv_diagnostic push
+#pragma nv_diag_suppress 1427
+#elif defined(__NVCOMPILER)
+#pragma diagnostic push
+#pragma diag_suppress offset_in_non_POD_nonstandard
+#endif  // defined(__INTEL_COMPILER)
+      constexpr std::size_t offset = offsetof(proxy<F>, ia_);
+#if defined(__INTEL_COMPILER)
+#pragma warning pop
+#elif defined(__GNUC__) || defined(__clang__)
+#pragma GCC diagnostic pop
+#elif defined(__NVCC__)
+#pragma nv_diagnostic pop
+#elif defined(__NVCOMPILER)
+#pragma diagnostic pop
+#endif  // defined(__INTEL_COMPILER)
       return reinterpret_cast<add_qualifier_t<proxy<F>, Q>>(
           *(reinterpret_cast<add_qualifier_ptr_t<std::byte, Q>>(
               static_cast<add_qualifier_ptr_t<
                   typename facade_traits<F>::indirect_accessor, Q>>(
-                      std::addressof(a))) - offsetof(proxy<F>, ia_)));
+                      std::addressof(a))) - offset));
     }
   }
 };

tests/CMakeLists.txt

@@ -23,6 +23,7 @@ add_executable(msft_proxy_tests
   proxy_invocation_tests.cpp
   proxy_lifetime_tests.cpp
   proxy_reflection_tests.cpp
+  proxy_regression_tests.cpp
   proxy_traits_tests.cpp
 )
 target_include_directories(msft_proxy_tests PRIVATE .)

tests/proxy_regression_tests.cpp

@@ -0,0 +1,19 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT License.
+
+#include <gtest/gtest.h>
+#include "proxy.h"
+
+// https://github.com/microsoft/proxy/issues/213
+TEST(ProxyRegressionTests, TestUnexpectedCompilerWarning) {
+  struct MyTrivialFacade : pro::facade_builder
+      ::add_convention<pro::operator_dispatch<"()">, void(), void() const>
+      ::support_copy<pro::constraint_level::trivial>
+      ::support_relocation<pro::constraint_level::trivial>
+      ::support_destruction<pro::constraint_level::trivial>
+      ::build {};
+  int side_effect = 0;
+  pro::proxy<MyTrivialFacade> p = pro::make_proxy<MyTrivialFacade>([&] { side_effect = 1; });
+  (*p)();
+  EXPECT_EQ(side_effect, 1);
+}