3 files changed, 22 insertions, 0 deletions
diff --git a/clang/lib/Headers/stddef.h b/clang/lib/Headers/stddef.h
index 52962248f67..c4861551dac 100644
--- a/clang/lib/Headers/stddef.h
+++ b/clang/lib/Headers/stddef.h
@@ -42,6 +42,17 @@ typedef __PTRDIFF_TYPE__ ptrdiff_t;
 typedef __SIZE_TYPE__ size_t;
 #endif
 
+/* ISO9899:2011 7.20 (C11 Annex K): Define risze_t if __STDC_WANT_LIB_EXT1__ is
+ * enabled. */
+#if (defined(__STDC_WANT_LIB_EXT1__) && __STDC_WANT_LIB_EXT1__ >= 1 && \
+     !defined(_RSIZE_T)) || __has_feature(modules)
+/* Always define rsize_t when modules are available. */
+#if !__has_feature(modules)
+#define _RSIZE_T
+#endif
+typedef __SIZE_TYPE__ rsize_t;
+#endif
+
 #ifndef __cplusplus
 /* Always define wchar_t when modules are available. */
 #if !defined(_WCHAR_T) || __has_feature(modules)
diff --git a/clang/lib/Headers/stdint.h b/clang/lib/Headers/stdint.h
index 051047f7d20..11529c0c674 100644
--- a/clang/lib/Headers/stdint.h
+++ b/clang/lib/Headers/stdint.h
@@ -667,6 +667,12 @@ typedef __UINTMAX_TYPE__ uintmax_t;
 #define PTRDIFF_MAX  __INTN_MAX(__PTRDIFF_WIDTH__)
 #define    SIZE_MAX __UINTN_MAX(__SIZE_WIDTH__)
 
+/* ISO9899:2011 7.20 (C11 Annex K): Define RSIZE_MAX if __STDC_WANT_LIB_EXT1__
+ * is enabled. */
+#if defined(__STDC_WANT_LIB_EXT1__) && __STDC_WANT_LIB_EXT1__ >= 1
+#define   RSIZE_MAX            (SIZE_MAX >> 1)
+#endif
+
 /* C99 7.18.2.5 Limits of greatest-width integer types. */
 #define INTMAX_MIN   __INTN_MIN(__INTMAX_WIDTH__)
 #define INTMAX_MAX   __INTN_MAX(__INTMAX_WIDTH__)
diff --git a/clang/test/Headers/c11.c b/clang/test/Headers/c11.c
index f65164d15c2..7d859fc5f0c 100644
--- a/clang/test/Headers/c11.c
+++ b/clang/test/Headers/c11.c
@@ -17,3 +17,8 @@ _Static_assert(__alignas_is_defined, "");
 _Static_assert(__alignof_is_defined, "");
 alignas(alignof(int)) char c[4];
 _Static_assert(__alignof(c) == 4, "");
+
+#define __STDC_WANT_LIB_EXT1__ 1
+#include <stddef.h>
+#include <stdint.h>
+rsize_t x = RSIZE_MAX;