Trim the compiler-appended NUL from wide/UTF string literals too (#5702)

With JSON_STRICT_NUL_HANDLING defined to 1, parsing a wide, UTF-16,
UTF-32, or (C++20) UTF-8 string literal (e.g. json::parse(L"[1]"))
failed with parse_error.101 at the terminating NUL of the literal,
and accept() returned false. The array overload of input_adapter()
only dropped the compiler-added trailing '\0' for arrays of char,
so for wchar_t, char16_t, char32_t, and char8_t arrays that
terminator was passed to the parser as data, which the macro then
rejected.

Broaden the trimming to every character type that a string literal
can use (char, wchar_t, char16_t, char32_t, and, since C++20,
char8_t). Arrays of any other element type (unsigned char,
std::uint8_t, ...), as used for CBOR/MessagePack, are unaffected: a
trailing zero byte there is still read as genuine data.

Fixes #5658.

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

include/nlohmann/detail/input/input_adapters.hpp

@@ -855,16 +855,28 @@ template<typename T, std::size_t N>
 auto input_adapter(T (&array)[N]) -> decltype(input_adapter(array, array + N)) // NOLINT(cppcoreguidelines-avoid-c-arrays,hicpp-avoid-c-arrays,modernize-avoid-c-arrays)
 {
 #if JSON_STRICT_NUL_HANDLING
-    // A `char` array from string-literal initialization (e.g. json::parse("123"))
-    // carries a trailing '\0' contributed by the compiler, not by the source
-    // text; drop exactly that one byte so it is not mistaken for real trailing
-    // data. Every other element type (unsigned char, std::uint8_t, ...) keeps
-    // the full extent unconditionally, since a trailing zero byte there is
-    // genuine data (e.g. CBOR/MessagePack). This intentionally does not
-    // strlen()-scan the array (as the pointer overload above does for a
-    // null-delimited string): for a `char` array that is not NUL-terminated
-    // within its bounds, that would read past the end of the array.
-    if (std::is_same<typename std::remove_cv<T>::type, char>::value && N > 0 && array[N - 1] == 0)
+    // A text-literal array from string-literal initialization (e.g.
+    // json::parse("123") or json::parse(L"123")) carries a trailing '\0'
+    // contributed by the compiler, not by the source text; drop exactly that
+    // one byte so it is not mistaken for real trailing data. This covers all
+    // character types that string literals can use: char, wchar_t, char16_t,
+    // char32_t, and (C++20) char8_t. Every other element type (unsigned char,
+    // std::uint8_t, ...) keeps the full extent unconditionally, since a
+    // trailing zero byte there is genuine data (e.g. CBOR/MessagePack). This
+    // intentionally does not strlen()-scan the array (as the pointer overload
+    // above does for a null-delimited string): for an array that is not
+    // NUL-terminated within its bounds, that would read past the end of the
+    // array.
+    using char_t = typename std::remove_cv<T>::type;
+    constexpr bool is_text_literal_type = std::is_same<char_t, char>::value
+                                          || std::is_same<char_t, wchar_t>::value
+                                          || std::is_same<char_t, char16_t>::value
+                                          || std::is_same<char_t, char32_t>::value
+#if defined(__cpp_char8_t)
+                                          || std::is_same<char_t, char8_t>::value
+#endif
+                                          ;
+    if (is_text_literal_type && N > 0 && array[N - 1] == 0)
     {
         return input_adapter(array, array + N - 1);
     }