diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 1eebf5ea6b3e3..ade3c33b3b944 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -96,6 +96,10 @@ C++ Specific Potentially Breaking Changes
   Clang as a compiler, but it may break assumptions in Clang-based tools
   iterating over the AST.
 
+- The warning `-Wenum-constexpr-conversion` is now also enabled by default on
+  system headers and macros. It will be turned into a hard (non-downgradable)
+  error in the next Clang release.
+
 ABI Changes in This Version
 ---------------------------
 - Following the SystemV ABI for x86-64, ``__int128`` arguments will no longer
diff --git a/clang/include/clang/Basic/DiagnosticASTKinds.td b/clang/include/clang/Basic/DiagnosticASTKinds.td
index d2656310e79c9..0019553233fde 100644
--- a/clang/include/clang/Basic/DiagnosticASTKinds.td
+++ b/clang/include/clang/Basic/DiagnosticASTKinds.td
@@ -405,7 +405,8 @@ def warn_fixedpoint_constant_overflow : Warning<
   InGroup<DiagGroup<"fixed-point-overflow">>;
 def warn_constexpr_unscoped_enum_out_of_range : Warning<
   "integer value %0 is outside the valid range of values [%1, %2] for the "
-  "enumeration type %3">, DefaultError, InGroup<DiagGroup<"enum-constexpr-conversion">>;
+  "enumeration type %3">, DefaultError, ShowInSystemHeader, ShowInSystemMacro,
+  InGroup<DiagGroup<"enum-constexpr-conversion">>;
 
 // This is a temporary diagnostic, and shall be removed once our
 // implementation is complete, and like the preceding constexpr notes belongs
diff --git a/clang/test/SemaCXX/Inputs/enum-constexpr-conversion-system-header.h b/clang/test/SemaCXX/Inputs/enum-constexpr-conversion-system-header.h
new file mode 100644
index 0000000000000..0850f3405eed3
--- /dev/null
+++ b/clang/test/SemaCXX/Inputs/enum-constexpr-conversion-system-header.h
@@ -0,0 +1,19 @@
+// System header for testing that -Wenum-constexpr-conversion leads to an error
+// when included in user code, or when the system macro is used.
+
+enum SystemEnum
+{
+    a = 0,
+    b = 1,
+};
+
+void testValueInRangeOfEnumerationValuesInSystemHeader()
+{
+    constexpr SystemEnum x1 = static_cast<SystemEnum>(123);
+    // expected-error@-1 {{integer value 123 is outside the valid range of values [0, 1] for the enumeration type 'SystemEnum'}}
+
+    const SystemEnum x2 = static_cast<SystemEnum>(123);  // ok, not a constant expression context
+}
+
+#define CONSTEXPR_CAST_TO_SYSTEM_ENUM_OUTSIDE_OF_RANGE \
+    constexpr SystemEnum system_enum = static_cast<SystemEnum>(123)
diff --git a/clang/test/SemaCXX/constant-expression-cxx11.cpp b/clang/test/SemaCXX/constant-expression-cxx11.cpp
index 89d1b3ea6de05..8fb994224853b 100644
--- a/clang/test/SemaCXX/constant-expression-cxx11.cpp
+++ b/clang/test/SemaCXX/constant-expression-cxx11.cpp
@@ -1,6 +1,6 @@
-// RUN: %clang_cc1 -std=c++23 -fsyntax-only -verify=expected,cxx20_23,cxx23    -triple x86_64-linux -Wno-string-plus-int -Wno-pointer-arith -Wno-zero-length-array -Wno-c99-designator -fcxx-exceptions -pedantic %s -Wno-comment -Wno-tautological-pointer-compare -Wno-bool-conversion
-// RUN: %clang_cc1 -std=c++20 -fsyntax-only -verify=expected,cxx11_20,cxx20_23 -triple x86_64-linux -Wno-string-plus-int -Wno-pointer-arith -Wno-zero-length-array -Wno-c99-designator -fcxx-exceptions -pedantic %s -Wno-comment -Wno-tautological-pointer-compare -Wno-bool-conversion
-// RUN: %clang_cc1 -std=c++11 -fsyntax-only -verify=expected,cxx11_20,cxx11    -triple x86_64-linux -Wno-string-plus-int -Wno-pointer-arith -Wno-zero-length-array -Wno-c99-designator -fcxx-exceptions -pedantic %s -Wno-comment -Wno-tautological-pointer-compare -Wno-bool-conversion
+// RUN: %clang_cc1 -std=c++23 -isystem %S/Inputs -fsyntax-only -verify=expected,cxx20_23,cxx23    -triple x86_64-linux -Wno-string-plus-int -Wno-pointer-arith -Wno-zero-length-array -Wno-c99-designator -fcxx-exceptions -pedantic %s -Wno-comment -Wno-tautological-pointer-compare -Wno-bool-conversion
+// RUN: %clang_cc1 -std=c++20 -isystem %S/Inputs -fsyntax-only -verify=expected,cxx11_20,cxx20_23 -triple x86_64-linux -Wno-string-plus-int -Wno-pointer-arith -Wno-zero-length-array -Wno-c99-designator -fcxx-exceptions -pedantic %s -Wno-comment -Wno-tautological-pointer-compare -Wno-bool-conversion
+// RUN: %clang_cc1 -std=c++11 -isystem %S/Inputs -fsyntax-only -verify=expected,cxx11_20,cxx11    -triple x86_64-linux -Wno-string-plus-int -Wno-pointer-arith -Wno-zero-length-array -Wno-c99-designator -fcxx-exceptions -pedantic %s -Wno-comment -Wno-tautological-pointer-compare -Wno-bool-conversion
 
 namespace StaticAssertFoldTest {
 
@@ -2449,6 +2449,8 @@ E2 testDefaultArgForParam(E2 e2Param = (E2)-1) { // ok, not a constant expressio
   return e2LocalInit;
 }
 
+#include <enum-constexpr-conversion-system-header.h>
+
 void testValueInRangeOfEnumerationValues() {
   constexpr E1 x1 = static_cast<E1>(-8);
   constexpr E1 x2 = static_cast<E1>(8);
@@ -2486,6 +2488,9 @@ void testValueInRangeOfEnumerationValues() {
   // expected-error@-1 {{integer value 2147483648 is outside the valid range of values [-2147483648, 2147483647] for the enumeration type 'EMaxInt'}}
 
   const NumberType neg_one = (NumberType) ((NumberType) 0 - (NumberType) 1); // ok, not a constant expression context
+
+  CONSTEXPR_CAST_TO_SYSTEM_ENUM_OUTSIDE_OF_RANGE;
+  // expected-error@-1 {{integer value 123 is outside the valid range of values [0, 1] for the enumeration type 'SystemEnum'}}
 }
 
 template<class T, unsigned size> struct Bitfield {