Read a std::u8string from a JSON string and store it as one in every configuration (#6075)

* Read a std::u8string from a JSON string and store it as one in every configuration

In C++20, json(std::u8string) was stored as a JSON string only when
std::filesystem::path support was available (the to_json overload sat
inside the JSON_HAS_FILESYSTEM guard), and there was no matching
from_json, so get<std::u8string>() threw type_error.302 ("type must be
array, but is string"). Without std::filesystem, the same expression
still produced an array of code units (the 3.12.0 behavior).

Move the to_json overload out of the filesystem guard so that a
std::u8string is a string in every configuration, build the value
directly from the code units (this also works for a custom string_t
such as the tests' alt_string), and add a from_json overload that reads
a JSON string. The array of numbers written by 3.12.0 is still accepted;
other types throw type_error.302 ("type must be string or array").
std::u8string_view and char8_t arrays are unchanged.

The tests now run in C++20/23 (the file mentions JSON_HAS_CPP_20/23),
also without std::filesystem (test-conversions2_no_filesystem), and with
ordered_json and a custom string type. The new behavior is documented.

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

* Fill the std::u8string with resize and indexing instead of push_back

The push_back loop is not vectorized: reading a 1 kB string took about
2.2 us against 0.39 us with resize and direct element access (clang -O2).

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

* Write the std::u8string from_json template header in the compact form

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

* Ignore libstdc++ char to char8_t sign change in the clang sanitizer job

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

---------

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

tests/src/unit-alt-string.cpp

@@ -539,4 +539,20 @@ TEST_CASE("alternative string type with json_view")
     }
 }
 
+// a std::u8string is a JSON string, whatever the string type of the JSON value
+// JSON_HAS_CPP_20 JSON_HAS_CPP_23 (do not remove; see note at top of file)
+#if defined(__cpp_lib_char8_t)
+TEST_CASE("alternative string type and std::u8string")
+{
+    const std::u8string s = u8"abc";
+    const alt_json j = s;
+    CHECK(j.is_string());
+    CHECK(j.get<alt_string>() == alt_string("abc"));
+    CHECK(j.get<std::u8string>() == s);
+
+    // the array of code units written by versions before 3.13.0
+    CHECK(alt_json::array({0x61, 0x62, 0x63}).get<std::u8string>() == s);
+}
+#endif
+
 DOCTEST_CLANG_SUPPRESS_WARNING_POP