Index: clang/lib/Headers/stdatomic.h
===================================================================
--- clang/lib/Headers/stdatomic.h
+++ clang/lib/Headers/stdatomic.h
@@ -40,6 +40,10 @@
 /* 7.17.2 Initialization */
 
 #define ATOMIC_VAR_INIT(value) (value)
+#if __STDC_VERSION__ >= 201710L || __cplusplus >= 202002L
+/* ATOMIC_VAR_INIT was deprecated in C17 and C++20. */
+#pragma clang deprecated(ATOMIC_VAR_INIT)
+#endif
 #define atomic_init __c11_atomic_init
 
 /* 7.17.3 Order and consistency */
@@ -149,6 +153,10 @@
 typedef struct atomic_flag { atomic_bool _Value; } atomic_flag;
 
 #define ATOMIC_FLAG_INIT { 0 }
+#if __cplusplus >= 202002L
+/* ATOMIC_FLAG_INIT was deprecated in C++20 but is not deprecated in C. */
+#pragma clang deprecated(ATOMIC_FLAG_INIT)
+#endif
 
 /* These should be provided by the libc implementation. */
 #ifdef __cplusplus
Index: clang/test/Headers/stdatomic-deprecations.c
===================================================================
--- /dev/null
+++ clang/test/Headers/stdatomic-deprecations.c
@@ -0,0 +1,16 @@
+// RUN: %clang_cc1 -fsyntax-only -std=c11 %s -verify=okay
+// RUN: %clang_cc1 -fsyntax-only -std=c17 %s -verify
+// RUN: %clang_cc1 -fsyntax-only -std=c++11 -x c++ %s -verify=okay
+// RUN: %clang_cc1 -fsyntax-only -std=c++20 -x c++ %s -verify=cxx,expected
+
+// okay-no-diagnostics
+
+#include <stdatomic.h>
+
+void func(void) {
+  (void)ATOMIC_VAR_INIT(12); // expected-warning {{macro 'ATOMIC_VAR_INIT' has been marked as deprecated}} \
+                             // expected-note@stdatomic.h:* {{macro marked 'deprecated' here}}
+  #if defined(ATOMIC_FLAG_INIT) // cxx-warning {{macro 'ATOMIC_FLAG_INIT' has been marked as deprecated}} \
+                                // cxx-note@stdatomic.h:* {{macro marked 'deprecated' here}}
+  #endif
+}