🐛 fix for #656

A complete rewrite of the string escape function. It now provides codepoint-to-\uxxxx escaping. Invalid UTF-8 byte sequences are not escaped, but copied as-is. I haven’t spent much time optimizing the code - but the library now agrees with Python on every single Unicode character’s escaping (see file test/data/json_nlohmann_tests/all_unicode_ascii.json).

Other minor changes: replaced "size_t" by "std::size_t"

test/src/unit-convenience.cpp

@@ -98,7 +98,9 @@ TEST_CASE("convenience functions")
         check_escaped("\x1d", "\\u001d");
         check_escaped("\x1e", "\\u001e");
         check_escaped("\x1f", "\\u001f");
-        check_escaped("\xA9", "\xA9");
-        check_escaped("\xA9", "\\u00a9", true);
+
+        // invalid UTF-8 characters
+        check_escaped("ä\xA9ü", "ä\xA9ü");
+        check_escaped("ä\xA9ü", "\\u00e4\xA9\\u00fc", true);
     }
 }

test/src/unit-inspection.cpp

@@ -28,6 +28,7 @@ SOFTWARE.
 
 #include "catch.hpp"
 
+#include <fstream>
 #include "json.hpp"
 using nlohmann::json;
 
@@ -252,9 +253,35 @@ TEST_CASE("object inspection")
 
         SECTION("dump with ensure_ascii and non-ASCII characters")
         {
-            CHECK(json("ä").dump(-1, ' ', true) == R"("\u00c3\u00a4")");
-            CHECK(json("Ö").dump(-1, ' ', true) == R"("\u00c3\u0096")");
-            CHECK(json("❤️").dump(-1, ' ', true) == R"("\u00e2\u009d\u00a4\u00ef\u00b8\u008f")");
+            CHECK(json("ä").dump(-1, ' ', true) == "\"\\u00e4\"");
+            CHECK(json("Ö").dump(-1, ' ', true) == "\"\\u00d6\"");
+            CHECK(json("❤️").dump(-1, ' ', true) == "\"\\u2764\\ufe0f\"");
+        }
+
+        SECTION("full Unicode escaping to ASCII")
+        {
+            SECTION("parsing yields the same JSON value")
+            {
+                std::ifstream f_escaped("test/data/json_nlohmann_tests/all_unicode_ascii.json");
+                std::ifstream f_unescaped("test/data/json_nlohmann_tests/all_unicode.json");
+
+                json j1 = json::parse(f_escaped);
+                json j2 = json::parse(f_unescaped);
+                CHECK(j1 == j2);
+            }
+
+            SECTION("dumping yields the same JSON text")
+            {
+                std::ifstream f_escaped("test/data/json_nlohmann_tests/all_unicode_ascii.json");
+                std::ifstream f_unescaped("test/data/json_nlohmann_tests/all_unicode.json");
+
+                json value = json::parse(f_unescaped);
+                std::string text = value.dump(4, ' ', true);
+
+                std::string expected((std::istreambuf_iterator<char>(f_escaped)),
+                                     std::istreambuf_iterator<char>());
+                CHECK(text == expected);
+            }
         }
 
         SECTION("serialization of discarded element")