[Language Conformance] Use constexpr restriction in jsoncpp (open-source-parsers#1005)

* use constexpr restriction in jsoncpp

* remove TODO comment

Author: dota17

include/json/value.h

@@ -203,31 +203,33 @@ class JSON_API Value {
   static Value const& nullSingleton();
 
   /// Minimum signed integer value that can be stored in a Json::Value.
-  static const LargestInt minLargestInt;
+  static constexpr LargestInt minLargestInt = LargestInt(~(LargestUInt(-1) / 2));
   /// Maximum signed integer value that can be stored in a Json::Value.
-  static const LargestInt maxLargestInt;
+  static constexpr LargestInt maxLargestInt = LargestInt(LargestUInt(-1) / 2);
   /// Maximum unsigned integer value that can be stored in a Json::Value.
-  static const LargestUInt maxLargestUInt;
+  static constexpr LargestUInt maxLargestUInt = LargestUInt(-1);
 
   /// Minimum signed int value that can be stored in a Json::Value.
-  static const Int minInt;
+  static constexpr Int minInt = Int(~(UInt(-1) / 2));
   /// Maximum signed int value that can be stored in a Json::Value.
-  static const Int maxInt;
+  static constexpr Int maxInt = Int(UInt(-1) / 2);
   /// Maximum unsigned int value that can be stored in a Json::Value.
-  static const UInt maxUInt;
+  static constexpr UInt maxUInt = UInt(-1);
 
 #if defined(JSON_HAS_INT64)
   /// Minimum signed 64 bits int value that can be stored in a Json::Value.
-  static const Int64 minInt64;
+  static constexpr Int64 minInt64 = Int64(~(UInt64(-1) / 2));
   /// Maximum signed 64 bits int value that can be stored in a Json::Value.
-  static const Int64 maxInt64;
+  static constexpr Int64 maxInt64 = Int64(UInt64(-1) / 2);
   /// Maximum unsigned 64 bits int value that can be stored in a Json::Value.
-  static const UInt64 maxUInt64;
+  static constexpr UInt64 maxUInt64 = UInt64(-1);
 #endif // defined(JSON_HAS_INT64)
-
   /// Default precision for real value for string representation.
-  static const UInt defaultRealPrecision;
-
+  static constexpr UInt defaultRealPrecision = 17;
+  // The constant is hard-coded because some compiler have trouble
+  // converting Value::maxUInt64 to a double correctly (AIX/xlC).
+  // Assumes that UInt64 is a 64 bits integer.
+  static constexpr double maxUInt64AsDouble = 18446744073709551615.0;
 // Workaround for bug in the NVIDIAs CUDA 9.1 nvcc compiler
 // when using gcc and clang backend compilers.  CZString
 // cannot be defined as private.  See issue #486

src/lib_json/json_reader.cpp

@@ -1548,12 +1548,11 @@ bool OurReader::decodeNumber(Token& token, Value& decoded) {
   if (isNegative)
     ++current;
 
-  // TODO(issue #960): Change to constexpr
-  static const auto positive_threshold = Value::maxLargestUInt / 10;
-  static const auto positive_last_digit = Value::maxLargestUInt % 10;
-  static const auto negative_threshold =
+  static constexpr auto positive_threshold = Value::maxLargestUInt / 10;
+  static constexpr auto positive_last_digit = Value::maxLargestUInt % 10;
+  static constexpr auto negative_threshold =
       Value::LargestUInt(Value::minLargestInt) / 10;
-  static const auto negative_last_digit =
+  static constexpr auto negative_last_digit =
       Value::LargestUInt(Value::minLargestInt) % 10;
 
   const auto threshold = isNegative ? negative_threshold : positive_threshold;

src/lib_json/json_value.cpp

@@ -88,23 +88,6 @@ Value const& Value::null = Value::nullSingleton();
 Value const& Value::nullRef = Value::nullSingleton();
 #endif
 
-const Int Value::minInt = Int(~(UInt(-1) / 2));
-const Int Value::maxInt = Int(UInt(-1) / 2);
-const UInt Value::maxUInt = UInt(-1);
-#if defined(JSON_HAS_INT64)
-const Int64 Value::minInt64 = Int64(~(UInt64(-1) / 2));
-const Int64 Value::maxInt64 = Int64(UInt64(-1) / 2);
-const UInt64 Value::maxUInt64 = UInt64(-1);
-// The constant is hard-coded because some compiler have trouble
-// converting Value::maxUInt64 to a double correctly (AIX/xlC).
-// Assumes that UInt64 is a 64 bits integer.
-static const double maxUInt64AsDouble = 18446744073709551615.0;
-#endif // defined(JSON_HAS_INT64)
-const LargestInt Value::minLargestInt = LargestInt(~(LargestUInt(-1) / 2));
-const LargestInt Value::maxLargestInt = LargestInt(LargestUInt(-1) / 2);
-const LargestUInt Value::maxLargestUInt = LargestUInt(-1);
-
-const UInt Value::defaultRealPrecision = 17;
 
 #if !defined(JSON_USE_INT64_DOUBLE_CONVERSION)
 template <typename T, typename U>