Cover every byte class in the ill-formed UTF-8 sweeps

Pinning the bytes around the invalid one to 0x80 lost coverage: with
error_handler_t::ignore/replace, the serializer re-reads the invalid
byte and decodes the following bytes, and its decoder distinguishes
the continuation classes 0x80-0x8F, 0x90-0x9F, and 0xA0-0xBF, as do
the lexer's range checks.

Iterate those positions over the first and last byte of each class
within the valid range instead (utils::utf8_continuation_bytes). The
invalid byte still takes all 256 values. Defining
JSON_TEST_UTF8_EXHAUSTIVE restores the full Cartesian product, with
the same assertion counts as before #5418.

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

tests/src/test_utils.hpp

@@ -8,6 +8,7 @@
 
 #pragma once
 
+#include <array> // array
 #include <cstdint> // uint8_t
 #include <cstddef> // size_t
 #include <fstream> // ifstream, istreambuf_iterator, ios
@@ -42,6 +43,33 @@ T next_integer_sample(T i, T last, T stride)
     return n < last ? n : last;
 }
 
+// UTF-8 continuation bytes in [lo, hi] that stand in for all of them in the
+// ill-formed UTF-8 tests. Both the lexer's range checks and the serializer's
+// decoder (detail::decode) only distinguish the classes 0x80..0x8F, 0x90..0x9F,
+// and 0xA0..0xBF, so the first and last byte of each class within [lo, hi]
+// exercise every behavior while a test sweeps another byte position through
+// all 256 values (#5418). Define JSON_TEST_UTF8_EXHAUSTIVE to get every byte.
+inline std::vector<int> utf8_continuation_bytes(int lo, int hi)
+{
+    std::vector<int> result;
+#ifdef JSON_TEST_UTF8_EXHAUSTIVE
+    for (int byte = lo; byte <= hi; ++byte)
+    {
+        result.push_back(byte);
+    }
+#else
+    static const std::array<int, 6> class_ends = {{0x80, 0x8F, 0x90, 0x9F, 0xA0, 0xBF}};
+    for (const int byte : class_ends)
+    {
+        if (lo <= byte && byte <= hi)
+        {
+            result.push_back(byte);
+        }
+    }
+#endif
+    return result;
+}
+
 inline std::vector<std::uint8_t> read_binary_file(const std::string& filename)
 {
     std::ifstream file(filename, std::ios::binary);