Index: include/clang/Analysis/Analyses/FormatString.h
===================================================================
--- include/clang/Analysis/Analyses/FormatString.h
+++ include/clang/Analysis/Analyses/FormatString.h
@@ -257,7 +257,12 @@
   const Kind K;
   QualType T;
   const char *Name = nullptr;
-  bool Ptr = false, IsSizeT = false;
+  bool Ptr = false;
+
+  /// The TypeKind identifies certain well-known types like size_t and
+  /// ptrdiff_t.
+  enum class TypeKind { DontCare, SizeT, PtrdiffT };
+  TypeKind TK = TypeKind::DontCare;
 
 public:
   ArgType(Kind K = UnknownTy, const char *N = nullptr) : K(K), Name(N) {}
@@ -267,7 +272,9 @@
   static ArgType Invalid() { return ArgType(InvalidTy); }
   bool isValid() const { return K != InvalidTy; }
 
-  bool isSizeT() const { return IsSizeT; }
+  bool isSizeT() const { return TK == TypeKind::SizeT; }
+
+  bool isPtrdiffT() const { return TK == TypeKind::PtrdiffT; }
 
   /// Create an ArgType which corresponds to the type pointer to A.
   static ArgType PtrTo(const ArgType& A) {
@@ -280,7 +287,15 @@
   /// Create an ArgType which corresponds to the size_t/ssize_t type.
   static ArgType makeSizeT(const ArgType &A) {
     ArgType Res = A;
-    Res.IsSizeT = true;
+    Res.TK = TypeKind::SizeT;
+    return Res;
+  }
+
+  /// Create an ArgType which corresponds to the ptrdiff_t/unsigned ptrdiff_t
+  /// type.
+  static ArgType makePtrdiffT(const ArgType &A) {
+    ArgType Res = A;
+    Res.TK = TypeKind::PtrdiffT;
     return Res;
   }
 
Index: lib/Analysis/PrintfFormatString.cpp
===================================================================
--- lib/Analysis/PrintfFormatString.cpp
+++ lib/Analysis/PrintfFormatString.cpp
@@ -472,7 +472,8 @@
                    ? ArgType(Ctx.LongLongTy, "__int64")
                    : ArgType(Ctx.IntTy, "__int32");
       case LengthModifier::AsPtrDiff:
-        return ArgType(Ctx.getPointerDiffType(), "ptrdiff_t");
+        return ArgType::makePtrdiffT(
+            ArgType(Ctx.getPointerDiffType(), "ptrdiff_t"));
       case LengthModifier::AsAllocate:
       case LengthModifier::AsMAllocate:
       case LengthModifier::AsWide:
@@ -505,7 +506,8 @@
                    ? ArgType(Ctx.UnsignedLongLongTy, "unsigned __int64")
                    : ArgType(Ctx.UnsignedIntTy, "unsigned __int32");
       case LengthModifier::AsPtrDiff:
-        return ArgType(Ctx.getUnsignedPointerDiffType(), "unsigned ptrdiff_t");
+        return ArgType::makePtrdiffT(
+            ArgType(Ctx.getUnsignedPointerDiffType(), "unsigned ptrdiff_t"));
       case LengthModifier::AsAllocate:
       case LengthModifier::AsMAllocate:
       case LengthModifier::AsWide:
Index: lib/Sema/SemaChecking.cpp
===================================================================
--- lib/Sema/SemaChecking.cpp
+++ lib/Sema/SemaChecking.cpp
@@ -6894,10 +6894,11 @@
     QualType CastTy;
     std::tie(CastTy, CastTyName) = shouldNotPrintDirectly(S.Context, IntendedTy, E);
     if (!CastTy.isNull()) {
-      // %zi/%zu are OK to use for NSInteger/NSUInteger of type int
+      // %zi/%zu and %td/%tu are OK to use for NSInteger/NSUInteger of type int
       // (long in ASTContext). Only complain to pedants.
       if ((CastTyName == "NSInteger" || CastTyName == "NSUInteger") &&
-          AT.isSizeT() && AT.matchesType(S.Context, CastTy))
+          (AT.isSizeT() || AT.isPtrdiffT()) &&
+          AT.matchesType(S.Context, CastTy))
         Pedantic = true;
       IntendedTy = CastTy;
       ShouldNotPrintDirectly = true;
Index: test/SemaObjC/format-size-spec-nsinteger.m
===================================================================
--- test/SemaObjC/format-size-spec-nsinteger.m
+++ test/SemaObjC/format-size-spec-nsinteger.m
@@ -1,5 +1,7 @@
 // RUN: %clang_cc1 -triple thumbv7-apple-ios -Wno-objc-root-class -fsyntax-only -verify -Wformat %s
 // RUN: %clang_cc1 -triple thumbv7-apple-ios -Wno-objc-root-class -fsyntax-only -verify -Wformat-pedantic -DPEDANTIC %s
+// RUN: %clang_cc1 -triple thumbv7k-apple-watchos2.0.0 -fsyntax-only -fblocks -verify %s
+// RUN: %clang_cc1 -triple thumbv7k-apple-watchos2.0.0 -fsyntax-only -fblocks -verify -Wformat-pedantic -DPEDANTIC %s
 
 #if !defined(PEDANTIC)
 // expected-no-diagnostics
@@ -8,9 +10,16 @@
 #if __LP64__
 typedef unsigned long NSUInteger;
 typedef long NSInteger;
+typedef long ptrdiff_t;
 #else
 typedef unsigned int NSUInteger;
 typedef int NSInteger;
+#if __is_target_os(watchos)
+  // Watch ABI uses long for ptrdiff_t.
+  typedef long ptrdiff_t;
+#else
+  typedef int ptrdiff_t;
+#endif
 #endif
 
 @class NSString;
@@ -28,3 +37,16 @@
   // expected-warning@-4 {{values of type 'NSUInteger' should not be used as format arguments; add an explicit cast to 'unsigned long' instead}}
 #endif
 }
+
+void testPtrdiffSpecifier(ptrdiff_t x) {
+  NSInteger i = 0;
+  NSUInteger j = 0;
+
+  NSLog(@"ptrdiff_t NSUinteger: %tu", j);
+  NSLog(@"ptrdiff_t NSInteger: %td", i);
+  NSLog(@"ptrdiff_t %tu, %td", x, x);
+#if __is_target_os(watchos) && defined(PEDANTIC)
+  // expected-warning@-4 {{values of type 'NSUInteger' should not be used as format arguments; add an explicit cast to 'unsigned long' instead}}
+  // expected-warning@-4 {{values of type 'NSInteger' should not be used as format arguments; add an explicit cast to 'long' instead}}
+#endif
+}