Throw an exception when trying to parse a negative number as an unsigned.

Fixing issue 859.

docs/Breaking-Changes.md

@@ -4,7 +4,7 @@
 
 ## HEAD ##
 
-_none_
+  * Throws an exception when trying to parse a negative number as an unsigned integer.
 
 ## 0.6.0 ##
 

include/yaml-cpp/node/convert.h

@@ -15,6 +15,7 @@
 #include <sstream>
 #include <type_traits>
 #include <vector>
+#include <type_traits>
 
 #include "yaml-cpp/binary.h"
 #include "yaml-cpp/node/impl.h"
@@ -133,6 +134,9 @@ inner_encode(const T& rhs, std::stringstream& stream){
       const std::string& input = node.Scalar();                            \
       std::stringstream stream(input);                                     \
       stream.unsetf(std::ios::dec);                                        \
+      if ((stream.peek() == '-') && std::is_unsigned<type>::value) {       \
+        return false;                                                      \
+      }                                                                    \
       if ((stream >> std::noskipws >> rhs) && (stream >> std::ws).eof()) { \
         return true;                                                       \
       }                                                                    \

test/integration/load_node_test.cpp

@@ -32,6 +32,11 @@ TEST(LoadNodeTest, NumericConversion) {
   EXPECT_EQ(-std::numeric_limits<float>::infinity(), node[4].as<float>());
   EXPECT_EQ(21, node[5].as<int>());
   EXPECT_EQ(13, node[6].as<int>());
+  // Throw exception: convert a negative number to an unsigned number.
+  EXPECT_THROW(node[7].as<unsigned>(), TypedBadConversion<unsigned int>);
+  EXPECT_THROW(node[7].as<unsigned short>(), TypedBadConversion<unsigned short>);
+  EXPECT_THROW(node[7].as<unsigned long>(), TypedBadConversion<unsigned long>);
+  EXPECT_THROW(node[7].as<unsigned long long>(), TypedBadConversion<unsigned long long>);
 }
 
 TEST(LoadNodeTest, Binary) {