diff --git a/llvm/lib/Support/JSON.cpp b/llvm/lib/Support/JSON.cpp
--- a/llvm/lib/Support/JSON.cpp
+++ b/llvm/lib/Support/JSON.cpp
@@ -509,13 +509,25 @@
     S.push_back(next());
   char *End;
   // Try first to parse as integer, and if so preserve full 64 bits.
-  // strtoll returns long long >= 64 bits, so check it's in range too.
-  auto I = std::strtoll(S.c_str(), &End, 10);
-  if (End == S.end() && I >= std::numeric_limits<int64_t>::min() &&
-      I <= std::numeric_limits<int64_t>::max()) {
+  // We check for errno for out of bounds errors and for End == S.end()
+  // to make sure that the numeric string is not malformed.
+  errno = 0;
+  int64_t I = std::strtoll(S.c_str(), &End, 10);
+  if (End == S.end() && errno != ERANGE) {
     Out = int64_t(I);
     return true;
   }
+  // strtroull has a special handling for negative numbers, but in this
+  // case we don't want to do that because negative numbers were already
+  // handled in the previous block.
+  if (First != '-') {
+    errno = 0;
+    uint64_t UI = std::strtoull(S.c_str(), &End, 10);
+    if (End == S.end() && errno != ERANGE) {
+      Out = UI;
+      return true;
+    }
+  }
   // If it's not an integer
   Out = std::strtod(S.c_str(), &End);
   return End == S.end() || parseError("Invalid JSON value (number?)");
@@ -914,4 +926,3 @@
     llvm_unreachable("json::Value format options should be an integer");
   json::OStream(OS, IndentAmount).value(E);
 }
-
diff --git a/llvm/unittests/Support/JSONTest.cpp b/llvm/unittests/Support/JSONTest.cpp
--- a/llvm/unittests/Support/JSONTest.cpp
+++ b/llvm/unittests/Support/JSONTest.cpp
@@ -362,19 +362,80 @@
   uint64_t Var = 3100100100;
   EXPECT_EQ(Val, Var);
 
+  Val = uint64_t{std::numeric_limits<uint64_t>::max()};
+  Var = std::numeric_limits<uint64_t>::max();
+  EXPECT_EQ(Val, Var);
+
   // Test the parse() part.
-  const char *Str = "4611686018427387905";
-  llvm::Expected<Value> Doc = parse(Str);
+  {
+    const char *Str = "4611686018427387905";
+    llvm::Expected<Value> Doc = parse(Str);
 
-  EXPECT_TRUE(!!Doc);
-  EXPECT_EQ(Doc->getAsInteger(), int64_t{4611686018427387905});
-  EXPECT_EQ(Doc->getAsUINT64(), uint64_t{4611686018427387905});
-
-  const char *Str2 = "-78278238238328222";
-  llvm::Expected<Value> Doc2 = parse(Str2);
-  EXPECT_TRUE(!!Doc2);
-  EXPECT_EQ(Doc2->getAsInteger(), int64_t{-78278238238328222});
-  EXPECT_EQ(Doc2->getAsUINT64(), llvm::None);
+    EXPECT_TRUE(!!Doc);
+    EXPECT_EQ(Doc->getAsInteger(), int64_t{4611686018427387905});
+    EXPECT_EQ(Doc->getAsUINT64(), uint64_t{4611686018427387905});
+  }
+
+  {
+    const char *Str = "-78278238238328222";
+    llvm::Expected<Value> Doc = parse(Str);
+
+    EXPECT_TRUE(!!Doc);
+    EXPECT_EQ(Doc->getAsInteger(), int64_t{-78278238238328222});
+    EXPECT_EQ(Doc->getAsUINT64(), llvm::None);
+  }
+
+  // Test with the largest 64 signed int
+  {
+    const char *Str = "9223372036854775807";
+    llvm::Expected<Value> Doc = parse(Str);
+
+    EXPECT_TRUE(!!Doc);
+    EXPECT_EQ(Doc->getAsInteger(), int64_t{9223372036854775807});
+    EXPECT_EQ(Doc->getAsUINT64(), uint64_t{9223372036854775807});
+  }
+
+  // Test with the largest 64 unsigned int
+  {
+    const char *Str = "18446744073709551615";
+    llvm::Expected<Value> Doc = parse(Str);
+
+    EXPECT_TRUE(!!Doc);
+    EXPECT_EQ(Doc->getAsInteger(), None);
+    EXPECT_EQ(Doc->getAsUINT64(), uint64_t{18446744073709551615u});
+  }
+
+  // Test with a number that is too big for 64 bits.
+  {
+    const char *Str = "184467440737095516150";
+    llvm::Expected<Value> Doc = parse(Str);
+
+    EXPECT_TRUE(!!Doc);
+    EXPECT_EQ(Doc->getAsInteger(), None);
+    EXPECT_EQ(Doc->getAsUINT64(), None);
+    // The number was parsed as a double
+    EXPECT_TRUE(!!Doc->getAsNumber());
+  }
+
+  // Test with a negative number that is too small for 64 bits.
+  {
+    const char *Str = "-18446744073709551615";
+    llvm::Expected<Value> Doc = parse(Str);
+
+    EXPECT_TRUE(!!Doc);
+    EXPECT_EQ(Doc->getAsInteger(), None);
+    EXPECT_EQ(Doc->getAsUINT64(), None);
+    // The number was parsed as a double
+    EXPECT_TRUE(!!Doc->getAsNumber());
+  }
+  // Test with a large number that is malformed.
+  {
+    const char *Str = "184467440737095516150.12.12";
+    llvm::Expected<Value> Doc = parse(Str);
+
+    EXPECT_EQ("[1:27, byte=27]: Invalid JSON value (number?)",
+              llvm::toString(Doc.takeError()));
+  }
 }
 
 // Sample struct with typical JSON-mapping rules.