diff --git a/include/xgboost/json_io.h b/include/xgboost/json_io.h
index e11545b04..3a73d170a 100644
--- a/include/xgboost/json_io.h
+++ b/include/xgboost/json_io.h
@@ -1,5 +1,5 @@
-/*!
- * Copyright (c) by Contributors 2019-2022
+/**
+ * Copyright 2019-2023, XGBoost Contributors
  */
 #ifndef XGBOOST_JSON_IO_H_
 #define XGBOOST_JSON_IO_H_
@@ -17,44 +17,26 @@
 #include <vector>
 
 namespace xgboost {
-namespace detail {
-// Whether char is signed is undefined, as a result we might or might not need
-// static_cast and std::to_string.
-template <typename Char, std::enable_if_t<std::is_signed<Char>::value>* = nullptr>
-std::string CharToStr(Char c) {
-  static_assert(std::is_same<Char, char>::value);
-  return std::string{c};
-}
-
-template <typename Char, std::enable_if_t<!std::is_signed<Char>::value>* = nullptr>
-std::string CharToStr(Char c) {
-  static_assert(std::is_same<Char, char>::value);
-  return (c <= static_cast<char>(127) ? std::string{c} : std::to_string(c));
-}
-}  // namespace detail
-
-/*
+/**
  * \brief A json reader, currently error checking and utf-8 is not fully supported.
  */
 class JsonReader {
+ public:
+  using Char = std::int8_t;
+
  protected:
-  size_t constexpr static kMaxNumLength =
-      std::numeric_limits<double>::max_digits10 + 1;
+  size_t constexpr static kMaxNumLength = std::numeric_limits<double>::max_digits10 + 1;
 
   struct SourceLocation {
    private:
-    size_t pos_ { 0 };  // current position in raw_str_
+    std::size_t pos_{0};  // current position in raw_str_
 
    public:
     SourceLocation() = default;
-    size_t  Pos()  const { return pos_; }
+    size_t Pos() const { return pos_; }
 
-    void Forward() {
-      pos_++;
-    }
-    void Forward(uint32_t n) {
-      pos_ += n;
-    }
+    void Forward() { pos_++; }
+    void Forward(uint32_t n) { pos_ += n; }
   } cursor_;
 
   StringView raw_str_;
@@ -62,7 +44,7 @@ class JsonReader {
  protected:
   void SkipSpaces();
 
-  char GetNextChar() {
+  Char GetNextChar() {
     if (XGBOOST_EXPECT((cursor_.Pos() == raw_str_.size()), false)) {
       return -1;
     }
@@ -71,24 +53,24 @@ class JsonReader {
     return ch;
   }
 
-  char PeekNextChar() {
+  Char PeekNextChar() {
     if (cursor_.Pos() == raw_str_.size()) {
       return -1;
     }
-    char ch = raw_str_[cursor_.Pos()];
+    Char ch = raw_str_[cursor_.Pos()];
     return ch;
   }
 
   /* \brief Skip spaces and consume next character. */
-  char GetNextNonSpaceChar() {
+  Char GetNextNonSpaceChar() {
     SkipSpaces();
     return GetNextChar();
   }
   /* \brief Consume next character without first skipping empty space, throw when the next
    *        character is not the expected one.
    */
-  char GetConsecutiveChar(char expected_char) {
-    char result = GetNextChar();
+  Char GetConsecutiveChar(char expected_char) {
+    Char result = GetNextChar();
     if (XGBOOST_EXPECT(result != expected_char, false)) { Expect(expected_char, result); }
     return result;
   }
@@ -96,7 +78,7 @@ class JsonReader {
   void Error(std::string msg) const;
 
   // Report expected character
-  void Expect(char c, char got) {
+  void Expect(Char c, Char got) {
     std::string msg = "Expecting: \"";
     msg += c;
     msg += "\", got: \"";
@@ -105,7 +87,7 @@ class JsonReader {
     } else if (got == 0) {
       msg += "\\0\"";
     } else {
-      msg += detail::CharToStr(got) + " \"";
+      msg += std::to_string(got) + " \"";
     }
     Error(msg);
   }
diff --git a/src/common/algorithm.h b/src/common/algorithm.h
index 739a84968..a34010cd0 100644
--- a/src/common/algorithm.h
+++ b/src/common/algorithm.h
@@ -14,7 +14,7 @@
 
 // clang with libstdc++ works as well
 #if defined(__GNUC__) && (__GNUC__ >= 4) && !defined(__sun) && !defined(sun) && \
-    !defined(__APPLE__) && __has_include(<omp.h>)
+    !defined(__APPLE__) && __has_include(<omp.h>) && __has_include(<parallel/algorithm>)
 #define GCC_HAS_PARALLEL 1
 #endif  // GLIC_VERSION
 
diff --git a/src/common/json.cc b/src/common/json.cc
index 8e2dd05ff..c3d61b47d 100644
--- a/src/common/json.cc
+++ b/src/common/json.cc
@@ -333,7 +333,7 @@ size_t constexpr JsonReader::kMaxNumLength;
 Json JsonReader::Parse() {
   while (true) {
     SkipSpaces();
-    char c = PeekNextChar();
+    auto c = PeekNextChar();
     if (c == -1) { break; }
 
     if (c == '{') {
@@ -408,13 +408,13 @@ void JsonReader::Error(std::string msg) const {
 }
 
 namespace {
-bool IsSpace(char c) { return c == ' ' || c == '\n' || c == '\r' || c == '\t'; }
+bool IsSpace(JsonReader::Char c) { return c == ' ' || c == '\n' || c == '\r' || c == '\t'; }
 }  // anonymous namespace
 
 // Json class
 void JsonReader::SkipSpaces() {
   while (cursor_.Pos() < raw_str_.size()) {
-    char c = raw_str_[cursor_.Pos()];
+    Char c = raw_str_[cursor_.Pos()];
     if (IsSpace(c)) {
       cursor_.Forward();
     } else {
@@ -436,12 +436,12 @@ void ParseStr(std::string const& str) {
 }
 
 Json JsonReader::ParseString() {
-  char ch { GetConsecutiveChar('\"') };  // NOLINT
+  Char ch { GetConsecutiveChar('\"') };  // NOLINT
   std::string str;
   while (true) {
     ch = GetNextChar();
     if (ch == '\\') {
-      char next = static_cast<char>(GetNextChar());
+      Char next{GetNextChar()};
       switch (next) {
         case 'r':  str += u8"\r"; break;
         case 'n':  str += u8"\n"; break;
@@ -466,8 +466,8 @@ Json JsonReader::ParseString() {
 }
 
 Json JsonReader::ParseNull() {
-  char ch = GetNextNonSpaceChar();
-  std::string buffer{ch};
+  Char ch = GetNextNonSpaceChar();
+  std::string buffer{static_cast<char>(ch)};
   for (size_t i = 0; i < 3; ++i) {
     buffer.push_back(GetNextChar());
   }
@@ -480,7 +480,7 @@ Json JsonReader::ParseNull() {
 Json JsonReader::ParseArray() {
   std::vector<Json> data;
 
-  char ch { GetConsecutiveChar('[') };  // NOLINT
+  Char ch { GetConsecutiveChar('[') };  // NOLINT
   while (true) {
     if (PeekNextChar() == ']') {
       GetConsecutiveChar(']');
@@ -503,7 +503,7 @@ Json JsonReader::ParseObject() {
 
   Object::Map data;
   SkipSpaces();
-  char ch = PeekNextChar();
+  auto ch = PeekNextChar();
 
   if (ch == '}') {
     GetConsecutiveChar('}');
@@ -652,7 +652,7 @@ Json JsonReader::ParseNumber() {
 
 Json JsonReader::ParseBoolean() {
   bool result = false;
-  char ch = GetNextNonSpaceChar();
+  Char ch = GetNextNonSpaceChar();
   std::string const t_value = u8"true";
   std::string const f_value = u8"false";
 
@@ -737,7 +737,8 @@ Json UBJReader::ParseArray() {
       case 'L':
         return ParseTypedArray<I64Array>(n);
       default:
-        LOG(FATAL) << "`" + std::string{type} + "` is not supported for typed array.";  // NOLINT
+        LOG(FATAL) << "`" + std::string{static_cast<char>(type)} +  // NOLINT
+                          "` is not supported for typed array.";
     }
   }
   std::vector<Json> results;
@@ -794,7 +795,7 @@ Json UBJReader::Load() {
 
 Json UBJReader::Parse() {
   while (true) {
-    char c = PeekNextChar();
+    auto c = PeekNextChar();
     if (c == -1) {
       break;
     }