Make JSONWriter's number handling locale-independent

Closes #3176

tests/SelfTest/IntrospectiveTests/Json.tests.cpp

@@ -12,6 +12,7 @@
 #include <catch2/internal/catch_jsonwriter.hpp>
 #include <catch2/matchers/catch_matchers_string.hpp>
 
+#include <locale>
 #include <sstream>
 
 namespace {
@@ -20,6 +21,27 @@ namespace {
         return os << "custom";
     }
 
+    // Obviously wrong numpunct if it is actually used by the JSON writer.
+    class TestNumpunct : public std::numpunct<char> {
+        char do_decimal_point() const override { return '?'; }
+        char do_thousands_sep() const override { return '!'; }
+        std::string do_grouping() const override { return "\1"; }
+        std::string do_truename() const override { return "real"; }
+        std::string do_falsename() const override { return "fake"; }
+    };
+
+    class LocaleGuard {
+        std::locale m_previous_locale;
+
+    public:
+        explicit LocaleGuard( std::locale const& locale ):
+            m_previous_locale{ std::locale() } {
+            std::locale::global( locale );
+        }
+
+        ~LocaleGuard() { std::locale::global( m_previous_locale ); }
+    };
+
 TEST_CASE( "JsonWriter", "[JSON][JsonWriter]" ) {
 
     std::stringstream stream;
@@ -152,6 +174,31 @@ TEST_CASE( "JsonWriter escapes characters in strings properly", "[JsonWriter]" )
     }
 }
 
+TEST_CASE( "JsonWriter serializes numbers independently of the global locale",
+           "[JsonWriter]" ) {
+    using Catch::Matchers::ContainsSubstring;
+
+    LocaleGuard locale_guard{ std::locale{ std::locale(), new TestNumpunct } };
+
+    std::stringstream sstream;
+    {
+        auto writer = Catch::JsonValueWriter{ sstream }.writeObject();
+        writer.write( "double" ).write( 1.5 );
+        writer.write( "int" ).write( 1234567 );
+        writer.write( "bool-1" ).write( true );
+        writer.write( "bool-2" ).write( false );
+        writer.write( "array" ).writeArray().write( 2.5 ).write( 1234567 );
+    }
+
+    REQUIRE_THAT( sstream.str(),
+                  ContainsSubstring( "\"double\": 1.5," ) &&
+                      ContainsSubstring( "\"int\": 1234567," ) &&
+                      ContainsSubstring( "\"bool-1\": true," ) &&
+                      ContainsSubstring( "\"bool-2\": false," ) &&
+                      ContainsSubstring(
+                          "\"array\": [\n    2.5,\n    1234567\n  ]\n}" ) );
+}
+
 TEST_CASE( "JsonWriter benchmarks", "[JsonWriter][!benchmark]" ) {
     const auto input_length = GENERATE( as<size_t>{}, 10, 100, 10'000 );
     std::string test_input( input_length, 'a' );