diff --git a/clang/test/Analysis/Inputs/system-header-simulator-cxx.h b/clang/test/Analysis/Inputs/system-header-simulator-cxx.h
--- a/clang/test/Analysis/Inputs/system-header-simulator-cxx.h
+++ b/clang/test/Analysis/Inputs/system-header-simulator-cxx.h
@@ -1253,3 +1253,11 @@
 };
 
 } // namespace std
+
+// C++17 std::byte
+#if __cplusplus >= 201703L
+namespace std {
+  enum class byte : unsigned char {};
+} // namespace std
+#endif
+
diff --git a/clang/test/Analysis/enum-cast-out-of-range-c++17.cpp b/clang/test/Analysis/enum-cast-out-of-range-c++17.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/Analysis/enum-cast-out-of-range-c++17.cpp
@@ -0,0 +1,26 @@
+// RUN: %clang_analyze_cc1 \
+// RUN:   -analyzer-checker=core,alpha.cplusplus.EnumCastOutOfRange \
+// RUN:   -std=c++17 -verify %s
+//
+// Test relaxed enum class initialization.
+//
+
+// expected-no-diagnostics
+
+#include "Inputs/system-header-simulator-cxx.h"
+
+void test_direct_list_init() {
+  std::byte b{0};       // OK
+  (void)b;
+}
+
+void test_copy_init() {
+  std::byte b = std::byte{0}; // OK
+  (void)b;
+}
+
+struct A { std::byte b; };
+void test_aggregate_copy_init() {
+  A a = {std::byte{42}}; // OK
+  (void)a;
+}