Index: lib/StaticAnalyzer/Checkers/CStringChecker.cpp
===================================================================
--- lib/StaticAnalyzer/Checkers/CStringChecker.cpp
+++ lib/StaticAnalyzer/Checkers/CStringChecker.cpp
@@ -661,7 +661,7 @@
     if (Recorded)
       return *Recorded;
   }
-  
+
   // Otherwise, get a new symbol and update the state.
   SValBuilder &svalBuilder = C.getSValBuilder();
   QualType sizeTy = svalBuilder.getContext().getSizeType();
@@ -669,8 +669,25 @@
                                                     MR, Ex, sizeTy,
                                                     C.blockCount());
 
-  if (!hypothetical)
+  if (!hypothetical) {
+    // In case of unbounded calls strlen etc bound the range to SIZE_MAX/4
+    // Fix PR16558
+    BasicValueFactory &BVF = svalBuilder.getBasicValueFactory();
+    const llvm::APSInt &maxValInt = BVF.getMaxValue(sizeTy);
+    NonLoc maxVal = svalBuilder.makeIntVal(maxValInt);
+    NonLoc Four = svalBuilder.makeIntVal(4, sizeTy).castAs<NonLoc>();
+    NonLoc maxLength = svalBuilder
+                         .evalBinOpNN(state, BO_Div, maxVal , Four, sizeTy).castAs<NonLoc>();
+    Optional<NonLoc> strLn = strLength.getAs<NonLoc>();
+    if (!strLn) {
+      state = state->set<CStringLength>(MR, strLength);
+      return strLength;
+    }
+    SVal evalLength = svalBuilder
+                        .evalBinOpNN(state, BO_LE, *strLn, maxLength, sizeTy);
+    state = state->assume(evalLength.castAs<DefinedOrUnknownSVal>(), true);
     state = state->set<CStringLength>(MR, strLength);
+  }
 
   return strLength;
 }
Index: test/Analysis/string.c
===================================================================
--- test/Analysis/string.c
+++ test/Analysis/string.c
@@ -23,7 +23,6 @@
 #else /* USE_BUILTINS */
 # define BUILTIN(f) f
 #endif /* USE_BUILTINS */
-
 #define NULL 0
 typedef typeof(sizeof(int)) size_t;
 
@@ -434,7 +433,7 @@
 		return;
 	if (strlen(src) != 2)
 		return;
-	strcat(dst, src); // expected-warning{{This expression will create a string whose length is too big to be represented as a size_t}}
+	strcat(dst, src);
 }
 
 
@@ -657,7 +656,7 @@
     return;
   if (strlen(src) != 2)
     return;
-  strncat(dst, src, 2); // expected-warning{{This expression will create a string whose length is too big to be represented as a size_t}}
+  strncat(dst, src, 2);
 }
 
 void strncat_zero(char *src) {