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-binary_writer_sinks.cpp

@@ -14,7 +14,9 @@ using nlohmann::json;
 #include <array>
 #include <cstdint>
 #include <limits>
+#include <memory>
 #include <sstream>
+#include <stdexcept>
 #include <string>
 #include <vector>
 
@@ -767,3 +769,69 @@ TEST_CASE("a write that throws leaves the same bytes in every kind of output")
     }
 }
 #endif
+
+#if !defined(JSON_NOEXCEPTION)
+namespace
+{
+
+// an adapter whose every write fails
+class failing_adapter : public nlohmann::detail::output_adapter_protocol<char>
+{
+  public:
+    [[noreturn]] void write_character(char /*c*/) override
+    {
+        throw std::runtime_error("write failed");
+    }
+
+    [[noreturn]] void write_characters(const char* /*s*/, std::size_t /*length*/) override
+    {
+        throw std::runtime_error("write failed");
+    }
+};
+
+} // namespace
+#endif
+
+TEST_CASE("output adapters")
+{
+    SECTION("writing a single character")
+    {
+        // the writers pass whole buffers on, but an adapter takes single
+        // characters as well
+        std::vector<char> as_vector;
+        nlohmann::detail::output_vector_adapter<char> vector_adapter(as_vector);
+        vector_adapter.write_character('a');
+        vector_adapter.write_character('b');
+        CHECK(as_vector == std::vector<char>({'a', 'b'}));
+
+        std::ostringstream as_stream;
+        nlohmann::detail::output_stream_adapter<char> stream_adapter(as_stream);
+        stream_adapter.write_character('c');
+        stream_adapter.write_character('d');
+        CHECK(as_stream.str() == "cd");
+
+        std::string as_string;
+        nlohmann::detail::output_string_adapter<char> string_adapter(as_string);
+        string_adapter.write_character('e');
+        string_adapter.write_character('f');
+        CHECK(as_string == "ef");
+    }
+
+#if !defined(JSON_NOEXCEPTION)
+    SECTION("a sink that is destroyed with characters left over")
+    {
+        // the destructor passes the pending characters on, as a write that
+        // threw leaves them there; the adapter failing then must not escape
+        {
+            nlohmann::detail::output_adapter_sink<char> sink(std::make_shared<failing_adapter>());
+            sink.write_character('x');
+        }
+
+        // while flush() reports the failure
+        nlohmann::detail::output_adapter_sink<char> sink(std::make_shared<failing_adapter>());
+        sink.write_character('x');
+        CHECK_THROWS_AS(sink.flush(), std::runtime_error&);
+        // (the character counts as written, so there is nothing left for the destructor)
+    }
+#endif
+}