From 038b164196a79014b8e741bfaad86e4914bbd07a Mon Sep 17 00:00:00 2001
From: "Stephan T. Lavavej" <stl@microsoft.com>
Date: Thu, 10 Sep 2026 12:17:00 -0700
Subject: [PATCH] Restore and test `_USE_MATH_DEFINES` support.

---
 stl/inc/__msvc_math.hpp                                    | 7 +++++++
 .../tests/P0533R9_constexpr_for_cmath_and_cstdlib/env.lst  | 4 ++++
 .../tests/P0533R9_constexpr_for_cmath_and_cstdlib/test.cpp | 6 ++++++
 3 files changed, 17 insertions(+)

diff --git a/stl/inc/__msvc_math.hpp b/stl/inc/__msvc_math.hpp
index 5934bd775f..457b20dab3 100644
--- a/stl/inc/__msvc_math.hpp
+++ b/stl/inc/__msvc_math.hpp
@@ -23,6 +23,13 @@ _STL_DISABLE_CLANG_WARNINGS
 #pragma push_macro("new")
 #undef new
 
+// https://learn.microsoft.com/cpp/c-runtime-library/math-constants
+// These non-Standard macros (`M_PI` etc.) are widely used, but at least they're opt-in.
+// The <numbers> header is a Standard alternative.
+#ifdef _USE_MATH_DEFINES
+#include <corecrt_math_defines.h>
+#endif
+
 extern "C++" {
 namespace _Msvc { // duplicate type traits not provided by <yvals_core.h>
     template <class _Ty>
diff --git a/tests/std/tests/P0533R9_constexpr_for_cmath_and_cstdlib/env.lst b/tests/std/tests/P0533R9_constexpr_for_cmath_and_cstdlib/env.lst
index a25d22d1bc..24d60708e6 100644
--- a/tests/std/tests/P0533R9_constexpr_for_cmath_and_cstdlib/env.lst
+++ b/tests/std/tests/P0533R9_constexpr_for_cmath_and_cstdlib/env.lst
@@ -24,6 +24,8 @@ PM_CL="/EHsc /Zc:cmath /std:c++latest /MD"
 PM_CL="/EHsc /Zc:cmath /std:c++latest /MDd"
 PM_CL="/EHsc /Zc:cmath /std:c++latest /MT"
 PM_CL="/EHsc /Zc:cmath /std:c++latest /MTd"
+# And test the non-Standard macros:
+PM_CL="/EHsc /Zc:cmath /std:c++latest /MTd /D_USE_MATH_DEFINES"
 
 # Explicitly disable libc-based math:
 PM_CL="/EHsc /Zc:cmath- /std:c++14 /MD"
@@ -46,3 +48,5 @@ PM_CL="/EHsc /Zc:cmath- /std:c++latest /MD"
 PM_CL="/EHsc /Zc:cmath- /std:c++latest /MDd"
 PM_CL="/EHsc /Zc:cmath- /std:c++latest /MT"
 PM_CL="/EHsc /Zc:cmath- /std:c++latest /MTd"
+# And test the non-Standard macros:
+PM_CL="/EHsc /Zc:cmath- /std:c++latest /MTd /D_USE_MATH_DEFINES"
diff --git a/tests/std/tests/P0533R9_constexpr_for_cmath_and_cstdlib/test.cpp b/tests/std/tests/P0533R9_constexpr_for_cmath_and_cstdlib/test.cpp
index b33dfc3d91..71b3c9f69b 100644
--- a/tests/std/tests/P0533R9_constexpr_for_cmath_and_cstdlib/test.cpp
+++ b/tests/std/tests/P0533R9_constexpr_for_cmath_and_cstdlib/test.cpp
@@ -690,3 +690,9 @@ int main() {
 #endif // ^^^ _HAS_CXX26 && defined(_MSVC_LIBC_MATH) ^^^
 #endif // ^^^ defined(__cpp_lib_constexpr_cmath) ^^^
 }
+
+#ifdef _USE_MATH_DEFINES
+static_assert(static_cast<int>(M_PI * 100.0) == 314, "Unexpected value for M_PI, reboot universe and try again.");
+#elif defined(M_PI)
+static_assert(false, "When _USE_MATH_DEFINES is not defined, M_PI should not be defined.");
+#endif // ^^^ defined(M_PI) ^^^