Index: lib/Analysis/PrintfFormatString.cpp
===================================================================
--- lib/Analysis/PrintfFormatString.cpp
+++ lib/Analysis/PrintfFormatString.cpp
@@ -466,8 +466,7 @@
       case LengthModifier::AsIntMax:
         return ArgType(Ctx.getIntMaxType(), "intmax_t");
       case LengthModifier::AsSizeT:
-        // FIXME: How to get the corresponding signed version of size_t?
-        return ArgType();
+        return ArgType(Ctx.getSignedSizeType(), "ssize_t");
       case LengthModifier::AsInt3264:
         return Ctx.getTargetInfo().getTriple().isArch64Bit()
                    ? ArgType(Ctx.LongLongTy, "__int64")
Index: test/FixIt/format.m
===================================================================
--- test/FixIt/format.m
+++ test/FixIt/format.m
@@ -229,6 +229,13 @@
   // CHECK: fix-it:"{{.*}}":{[[@LINE-2]]:11-[[@LINE-2]]:14}:"%+ld"
 }
 
+void testSizeTypes() {
+  printf("%zu", 0.f); // expected-warning{{format specifies type 'size_t' (aka 'unsigned long') but the argument has type 'float'}}
+  // CHECK: fix-it:"{{.*}}":{[[@LINE-1]]:11-[[@LINE-1]]:14}:"%f"
+  printf("%zd", 0.f); // expected-warning{{format specifies type 'ssize_t' (aka 'long') but the argument has type 'float'}}
+  // CHECK: fix-it:"{{.*}}":{[[@LINE-1]]:11-[[@LINE-1]]:14}:"%f"
+}
+
 void testEnum() {
   typedef enum {
     ImplicitA = 1,