Raise line coverage to 99.9 %: fix the measurement, remove unreachable code, add tests (#5997)

* CI: measure line coverage without lcov's unexecuted-block adjustment

Drop geninfo_unexecuted_blocks=1 from the lcov capture of ci_test_coverage:
it reset every line of an inlined or templated function to 0 as soon as one
copy had an unexecuted block, which understated the line coverage by about
three points.

Add -fvar-tracking-assignments to both coverage builds. At -O0, GCC inlines
always_inline functions before it places the coverage counters, so the call
sites of such helpers (put_literal() and others) got no line record and
showed as 0 in an -O3 translation unit that never reaches them. With the
flag, the call sites carry counts in every translation unit. The generated
code is identical.

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

* Remove unreachable code found by the coverage analysis

Remove code that no input can reach, and mark the lines that cannot be
covered with LCOV_EXCL_LINE:

- BJData writer: the dry_run parameter of the ND-array element writers was
  always true; replace them by range checks.
- BON8 writer: call throw_integer_out_of_range() directly instead of
  write_bon8_value() for values that always throw.
- binary_writer: integer_bits() is only called for integers of up to 64 bits.
- serializer, view serializer, lexer: replace guards that the callers make
  unreachable by assertions or by the simpler code (surrogate pairs under
  ensure_ascii, next_unget, '-' at the end of the buffer).
- json_view: drop unreachable branches in scalar(), set(), check_utf8_scan(),
  append_input_impl() and unescape().
- Mark exception-cleanup braces, 2 GiB limits, big-endian and 32-bit code, and
  fall-through case labels with LCOV_EXCL_LINE.

No behavior changes for any input that reaches these functions.

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

* Tests: cover the remaining lines of the library headers

Add tests for the lines of include/nlohmann that were not covered: the
binary readers and writers, the lexer and locale paths, the json_view
modules, the custom number, key and array types, json_pointer, json_patch,
diff and the iterators.

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

* Fix test failures in the coverage tests

- unit-custom-object-key-type: remove the unused make_strict_key(strict_key)
  overload, the unused range constructor and default constructor, and guard
  data() with JSON_DIAGNOSTICS (unused-function/-member-function errors with
  GCC and clang -Weverything).
- unit-json_view_compare: move the two new sections to the new
  unit-json_view_compare_values.cpp, as the object file got too large for the
  MinGW assembler again, and use a std::map with std::greater instead of
  std::unordered_map, which does not accept the incomplete basic_json
  (clang 3.9, icpc).
- unit-json_view_dump: rename the variables that hid outer ones (MSVC C4456).
- unit-stream-bulk-scan: restore the real size of the oversized get area
  before the adapter is destroyed (JSON_PRECISE_STREAM_POSITION reads the
  character at the read position), and skip the test where the streambuf
  cannot hold a get area larger than an int (MSVC).
- unit-regression1: with JSON_NO_THREAD_LOCAL, copies are built member by
  member and keep correct timestamps, so check the corresponding outcome.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01N5NasAgbDGG28MqXjQvLqp
Signed-off-by: Claude <noreply@anthropic.com>

* Return std::size_t from the hash helper in the ordered_json test

MSVC (Win32) rejected the implicit conversion of the unsigned long long
product to the 32-bit std::size_t (C4244 as an error). Also note when
JSON_NO_THREAD_LOCAL is set in the regression test.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01N5NasAgbDGG28MqXjQvLqp
Signed-off-by: Claude <noreply@anthropic.com>

---------

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
Signed-off-by: Claude <noreply@anthropic.com>
Co-authored-by: Claude <noreply@anthropic.com>

tests/src/unit-json_view_dump_escape.cpp

@@ -214,6 +214,24 @@ TEST_CASE("json_view dump: escaping, nesting, and output buffers")
         CHECK(d.root()["a"].dump(-1, ' ', true).size() == 2 + (5000 * 6));
     }
 
+    SECTION("the estimate for a value that is followed by many decoded strings")
+    {
+        // (the end of the value in the source is not known when the nodes after it have decoded strings)
+        std::string text = "[7";
+        for (int i = 0; i < 20; ++i)
+        {
+            text += R"(,"\n")";
+        }
+        text += ",8]";
+        const json_document d = json_document::parse(text);
+        CHECK(d.root()[0].dump() == "7");
+        CHECK(d.root()[1].dump() == R"("\n")");
+        CHECK(d.root().dump() == json::parse(text).dump());
+        // the same with a container
+        const json_document nested = json_document::parse(R"([[7],)" + text.substr(3));
+        CHECK(nested.root()[0].dump() == "[7]");
+    }
+
     SECTION("ensure_ascii with strings of the source, decoded strings, and edited strings")
     {
         // plain ASCII, non-ASCII, escaped (decoded), and mixed strings, as values and as keys;
@@ -259,6 +277,28 @@ TEST_CASE("json_view dump: escaping, nesting, and output buffers")
         }
     }
 
+    SECTION("the length of the run that ensure_ascii copies")
+    {
+        // (dump() only asks for strings with a byte to escape, so a run that ends the string is checked here)
+        for (const std::size_t length :
+                {
+                    std::size_t{0}, std::size_t{1}, std::size_t{15}, std::size_t{16}, std::size_t{17}, std::size_t{31}, std::size_t{32}, std::size_t{48}, std::size_t{50}
+                })
+        {
+            CAPTURE(length)
+            // a run of printable characters, followed by a byte that is not copied and more text
+            std::string text(length, 'a');
+            text += "\"abcdefghijklmnopqrstuvwxyz";
+            const auto* const bytes = reinterpret_cast<const unsigned char*>(text.data()); // NOLINT(cppcoreguidelines-pro-type-reinterpret-cast)
+            const auto* const end = bytes + text.size();
+            CHECK(nlohmann::detail::view::ascii_copyable_run(bytes, length, end) == length);
+            CHECK(nlohmann::detail::view::ascii_copyable_run(bytes, length + 1, end) == length);
+            CHECK(nlohmann::detail::view::ascii_copyable_run(bytes, length + 5, end) == length);
+            // the end of the buffer right after the run
+            CHECK(nlohmann::detail::view::ascii_copyable_run(bytes, length, bytes + length) == length);
+        }
+    }
+
     SECTION("ensure_ascii with DEL or a non-ASCII character at every position")
     {
         // the check whether a string of the source has no byte >= 0x7F reads