diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -237,6 +237,13 @@
 WebAssembly Support
 ^^^^^^^^^^^^^^^^^^^
 
+AVR Support
+^^^^^^^^^^^
+- The definition of ``USHRT_MAX`` in the freestanding ``<limits.h>`` no longer
+  overflows on AVR (where ``sizeof(int) == sizeof(unsigned short)``).  The type
+  of ``USHRT_MAX`` is now ``unsigned int`` instead of ``int``, as required by
+  the C standard.
+
 DWARF Support in Clang
 ----------------------
 
diff --git a/clang/lib/Headers/limits.h b/clang/lib/Headers/limits.h
--- a/clang/lib/Headers/limits.h
+++ b/clang/lib/Headers/limits.h
@@ -52,7 +52,11 @@
 #define LONG_MIN  (-__LONG_MAX__ -1L)
 
 #define UCHAR_MAX (__SCHAR_MAX__*2  +1)
-#define USHRT_MAX (__SHRT_MAX__ *2  +1)
+#if __SHRT_WIDTH__ < __INT_WIDTH__
+#define USHRT_MAX (__SHRT_MAX__ * 2 + 1)
+#else
+#define USHRT_MAX (__SHRT_MAX__ * 2U + 1U)
+#endif
 #define UINT_MAX  (__INT_MAX__  *2U +1U)
 #define ULONG_MAX (__LONG_MAX__ *2UL+1UL)
 
diff --git a/clang/test/Headers/limits.cpp b/clang/test/Headers/limits.cpp
--- a/clang/test/Headers/limits.cpp
+++ b/clang/test/Headers/limits.cpp
@@ -3,14 +3,40 @@
 // RUN: %clang_cc1 -std=c++11 -ffreestanding -fsyntax-only -verify %s
 // RUN: %clang_cc1 -std=c17 -ffreestanding -fsyntax-only -verify -x c %s
 // RUN: %clang_cc1 -std=c2x -ffreestanding -fsyntax-only -verify -x c %s
+
+// Specifically test 16-bit int platforms.
+// RUN: %clang_cc1 -triple=avr -ffreestanding -fsyntax-only -verify -x c %s
+// RUN: %clang_cc1 -triple=avr -std=c++11 -ffreestanding -fsyntax-only -verify %s
+
 // expected-no-diagnostics
 
 #include <limits.h>
 
+#if __cplusplus
+#define EXPR_TYPE_IS(EXPR, TYP) __is_same(__typeof(EXPR), TYP)
+#else
+#define EXPR_TYPE_IS(EXPR, TYP) _Generic(EXPR, TYP: 1, default: 0)
+#endif
+
 _Static_assert(SCHAR_MAX == -(SCHAR_MIN+1), "");
+_Static_assert(EXPR_TYPE_IS(SCHAR_MAX, int), "");
+#if SCHAR_MAX
+#endif
+
 _Static_assert(SHRT_MAX == -(SHRT_MIN+1), "");
+_Static_assert(EXPR_TYPE_IS(SHRT_MAX, int), "");
+#if SHRT_MAX
+#endif
+
 _Static_assert(INT_MAX == -(INT_MIN+1), "");
+_Static_assert(EXPR_TYPE_IS(INT_MAX, int), "");
+#if INT_MAX
+#endif
+
 _Static_assert(LONG_MAX == -(LONG_MIN+1L), "");
+_Static_assert(EXPR_TYPE_IS(LONG_MAX, long), "");
+#if LONG_MAX
+#endif
 
 _Static_assert(SCHAR_MAX == UCHAR_MAX/2, "");
 _Static_assert(SHRT_MAX == USHRT_MAX/2, "");
@@ -18,26 +44,84 @@
 _Static_assert(LONG_MAX == ULONG_MAX/2, "");
 
 _Static_assert(SCHAR_MIN == -SCHAR_MAX-1, "");
+_Static_assert(EXPR_TYPE_IS(SCHAR_MIN, int), "");
+#if SCHAR_MIN
+#endif
+
 _Static_assert(SHRT_MIN == -SHRT_MAX-1, "");
+_Static_assert(EXPR_TYPE_IS(SHRT_MIN, int), "");
+#if SHRT_MIN
+#endif
+
 _Static_assert(INT_MIN == -INT_MAX-1, "");
+_Static_assert(EXPR_TYPE_IS(INT_MIN, int), "");
+#if INT_MIN
+#endif
+
 _Static_assert(LONG_MIN == -LONG_MAX-1L, "");
+_Static_assert(EXPR_TYPE_IS(LONG_MIN, long), "");
+#if LONG_MIN
+#endif
 
 _Static_assert(UCHAR_MAX == (unsigned char)~0ULL, "");
+_Static_assert(UCHAR_MAX <= INT_MAX ?
+                 EXPR_TYPE_IS(UCHAR_MAX, int) :
+                 EXPR_TYPE_IS(UCHAR_MAX, unsigned int), "");
+#if UCHAR_MAX
+#endif
+
 _Static_assert(USHRT_MAX == (unsigned short)~0ULL, "");
+_Static_assert(USHRT_MAX <= INT_MAX ?
+                 EXPR_TYPE_IS(USHRT_MAX, int) :
+                 EXPR_TYPE_IS(USHRT_MAX, unsigned int), "");
+#if USHRT_MAX
+#endif
+
 _Static_assert(UINT_MAX == (unsigned int)~0ULL, "");
+_Static_assert(EXPR_TYPE_IS(UINT_MAX, unsigned int), "");
+#if UINT_MAX
+#endif
+
 _Static_assert(ULONG_MAX == (unsigned long)~0ULL, "");
+_Static_assert(EXPR_TYPE_IS(ULONG_MAX, unsigned long), "");
+#if ULONG_MAX
+#endif
 
 _Static_assert(MB_LEN_MAX >= 1, "");
+#if MB_LEN_MAX
+#endif
 
 _Static_assert(CHAR_BIT >= 8, "");
+#if CHAR_BIT
+#endif
 
 _Static_assert(CHAR_MIN == (((char)-1 < (char)0) ? -CHAR_MAX-1 : 0), "");
+_Static_assert(EXPR_TYPE_IS(CHAR_MIN, int), "");
+#if CHAR_MIN
+#endif
+
 _Static_assert(CHAR_MAX == (((char)-1 < (char)0) ? -(CHAR_MIN+1) : (char)~0ULL), "");
+_Static_assert(CHAR_MAX <= INT_MAX ?
+                 EXPR_TYPE_IS(CHAR_MAX, int) :
+                 EXPR_TYPE_IS(CHAR_MAX, unsigned int), "");
+#if CHAR_MAX
+#endif
 
 #if __STDC_VERSION__ >= 199901 || __cplusplus >= 201103L
 _Static_assert(LLONG_MAX == -(LLONG_MIN+1LL), "");
+_Static_assert(EXPR_TYPE_IS(LLONG_MAX, long long), "");
+#if LLONG_MAX
+#endif
+
 _Static_assert(LLONG_MIN == -LLONG_MAX-1LL, "");
+#if LLONG_MIN
+#endif
+_Static_assert(EXPR_TYPE_IS(LLONG_MIN, long long), "");
+
 _Static_assert(ULLONG_MAX == (unsigned long long)~0ULL, "");
+_Static_assert(EXPR_TYPE_IS(ULLONG_MAX, unsigned long long), "");
+#if ULLONG_MAX
+#endif
 #else
 int LLONG_MIN, LLONG_MAX, ULLONG_MAX; // Not defined.
 #endif
@@ -47,35 +131,61 @@
 #if __STDC_VERSION__ >= 202000L
 /* Validate the standard requirements. */
 _Static_assert(BOOL_WIDTH >= 1);
+#if BOOL_WIDTH
+#endif
 
 _Static_assert(CHAR_WIDTH == CHAR_BIT);
 _Static_assert(CHAR_WIDTH / CHAR_BIT == sizeof(char));
+#if CHAR_WIDTH
+#endif
 _Static_assert(SCHAR_WIDTH == CHAR_BIT);
 _Static_assert(SCHAR_WIDTH / CHAR_BIT == sizeof(signed char));
+#if SCHAR_WIDTH
+#endif
 _Static_assert(UCHAR_WIDTH == CHAR_BIT);
 _Static_assert(UCHAR_WIDTH / CHAR_BIT == sizeof(unsigned char));
+#if UCHAR_WIDTH
+#endif
 
 _Static_assert(USHRT_WIDTH >= 16);
 _Static_assert(USHRT_WIDTH / CHAR_BIT == sizeof(unsigned short));
+#if USHRT_WIDTH
+#endif
 _Static_assert(SHRT_WIDTH == USHRT_WIDTH);
 _Static_assert(SHRT_WIDTH / CHAR_BIT == sizeof(signed short));
+#if SHRT_WIDTH
+#endif
 
 _Static_assert(UINT_WIDTH >= 16);
 _Static_assert(UINT_WIDTH / CHAR_BIT == sizeof(unsigned int));
+#if UINT_WIDTH
+#endif
 _Static_assert(INT_WIDTH == UINT_WIDTH);
 _Static_assert(INT_WIDTH / CHAR_BIT == sizeof(signed int));
+#if INT_WIDTH
+#endif
 
 _Static_assert(ULONG_WIDTH >= 32);
 _Static_assert(ULONG_WIDTH / CHAR_BIT == sizeof(unsigned long));
+#if ULONG_WIDTH
+#endif
 _Static_assert(LONG_WIDTH == ULONG_WIDTH);
 _Static_assert(LONG_WIDTH / CHAR_BIT == sizeof(signed long));
+#if LONG_WIDTH
+#endif
 
 _Static_assert(ULLONG_WIDTH >= 64);
 _Static_assert(ULLONG_WIDTH / CHAR_BIT == sizeof(unsigned long long));
+#if ULLONG_WIDTH
+#endif
 _Static_assert(LLONG_WIDTH == ULLONG_WIDTH);
 _Static_assert(LLONG_WIDTH / CHAR_BIT == sizeof(signed long long));
+#if LLONG_WIDTH
+#endif
 
 _Static_assert(BITINT_MAXWIDTH >= ULLONG_WIDTH);
+#if BITINT_MAXWIDTH
+#endif
 #else
 /* None of these are defined. */
 int BOOL_WIDTH, CHAR_WIDTH, SCHAR_WIDTH, UCHAR_WIDTH, USHRT_WIDTH, SHRT_WIDTH,