Add tests for uncovered code paths (#5581)

* Add tests for uncovered code paths

Cover code the test suite did not reach, found from the Coveralls report
of develop and a local coverage run of HEAD:

- dump() of every kind of value below the bound of the recursive descent
  (pretty-printed objects, binary values, discarded values, scalars), and
  flushes of the escape and write buffers mid-string and mid-binary
- the iterative comparison: objects with different keys, containers that
  are a prefix of each other, and elements that cannot be ordered, each
  both at the top level and below the nesting bound
- SAX handlers that stop at any event, including the end of a nested
  container, in the BSON, CBOR, MessagePack, UBJSON and BJData readers
- from_bson/cbor/msgpack/ubjson/bjdata returning a discarded value
  through the iterator and pointer overloads
- JSON Patch, diff, merge_patch and update(..., true) on ordered_json
- smaller gaps: get_allocator(), to_ubjson/to_bjdata into a string,
  value() with an unresolvable JSON pointer, integer/float comparison
  below the integer range and with negative fractions, conversion to a
  custom binary type, std::formatter::parse on a spec without '}',
  unescape() of a lone '~', and the callback parser's start_array()

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Cover more paths that were thought unreachable

- parse_float_fast() declining malformed or inexact input, called
  directly since the lexer only passes well-formed numbers to it
- a UTF-16 high surrogate followed by a unit above the low surrogates
- self-assignment of a const_iterator
- a truncated CBOR string read through non-contiguous iterators
- serializing a long double under the de_DE locale, which undoes the
  locale's decimal point and thousands separator
- values read from a binary format carrying no diagnostic positions,
  with and without a parser callback

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Fix the CI failures of the new coverage tests

- declare the self-assignment reference const (misc-const-correctness)
- expect the (/path) prefix that JSON_DIAGNOSTICS adds to the messages
  of the failing ordered_json patch operations

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Expect the byte range JSON_DIAGNOSTIC_POSITIONS adds to the patch errors

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Build the expected dump of the nested-object test with +=

clang-tidy (performance-inefficient-string-concatenation) reported the
chain of operator+ calls that assembled the expected indented output.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Compare the BJData and UBJSON test outputs byte by byte

Building a std::string from the byte vector converts each byte
implicitly, which -fsanitize=integer reports for bytes of 0x80 and
above (ci_test_clang_sanitizer).

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

---------

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

tests/src/unit-diagnostic-positions.cpp

@@ -156,3 +156,38 @@ TEST_CASE("Better diagnostics with positions")
 #endif
     }
 }
+
+TEST_CASE("values read from a binary format have no positions")
+{
+    // only the JSON lexer knows where a value started and ended
+    const json source = {{"a", {1, "x", json::binary({1})}}, {"b", {{"c", true}}}, {"d", nullptr}, {"e", 1.5}};
+    const std::vector<std::uint8_t> cbor = json::to_cbor(source);
+
+    const auto check_no_positions = [](const json & j)
+    {
+        CHECK(j.start_pos() == std::string::npos);
+        CHECK(j.end_pos() == std::string::npos);
+        CHECK(j.at("a").start_pos() == std::string::npos);
+        CHECK(j.at("a").at(1).end_pos() == std::string::npos);
+        CHECK(j.at("b").at("c").start_pos() == std::string::npos);
+    };
+
+    SECTION("DOM parser")
+    {
+        const json j = json::from_cbor(cbor);
+        CHECK(j == source);
+        check_no_positions(j);
+    }
+
+    SECTION("DOM parser with a callback")
+    {
+        json j;
+        nlohmann::detail::json_sax_dom_callback_parser<json, decltype(nlohmann::detail::input_adapter(cbor))> sdp(j, [](int /*unused*/, json::parse_event_t /*unused*/, const json& /*unused*/) noexcept
+        {
+            return true;
+        });
+        CHECK(json::sax_parse(cbor, &sdp, json::input_format_t::cbor));
+        CHECK(j == source);
+        check_no_positions(j);
+    }
+}