And return false in that case, instead of returning 0x3f...
Instead of returning the encoding of the current substitution character. This allows a robust check for the failure case. The substitution character (especially the default of "?") is also a valid output of mb_chr() for a valid input (for "?" that would be 0x3f), so it's a bad choice for an error value.
Previously mb_chr() had two different encoding-dependent behaviors: * For "Unicode-encodings" it took a Unicode codepoint and returned its encoded representation. * Otherwise it returned a big-endian binary encoding of the passed integer. Now the input is always interpreted as a Unicode codepoint. If a big-endian binary encoding is what you want, you don't need mbstring to implement that.
The introduced checks were not correct in two respects: * It was checked whether the source encoding of the string matches the internal encoding, while the actually relevant encoding is the *target* encoding. * Even if the correct encoding is used, the checks are still too conservative. Just because something is not a "Unicode-encoding" does not mean that it does not map any non-ASCII characters. I've reverted the added checks and instead adjusted mbfl_convert to first try to use the provided substitution character and if that fails, perform the fallback to '?' at that point. This means that any codepoint mapped in the target encoding should now be correctly supported and anything else should fall back to '?'.
The introduced checks did not treat "non-Unicode" encodings correctly, because they treated the passed integer as encoded in the internal encoding in that case, while in actuality the substitute character is always a Unicode codepoint. Additionally checking the codepoint against the internal encoding is not correct in any case, because the substitution character must be mapped in the *target* encoding of the conversion, which does not necessarily coincide with the internal encoding (the internal encoding is the default *source* encoding, not *target* encoding). This reverts the checks back to simple range checks, but in a way that still resolves #69079: Characters outside the Basic Multilingual Plane are now accepted and Surrogate Codepoints are rejected. A distinction between UTF-8 and non-UTF-8 encodings is not made for surrogate checks (as in the original patch), as surrogates are always illegal on their own. Specifying a surrogate as substitution character would only make sense if you could specify a substitution string with more than one character -- however we do not support that.
ucgendat.c was assuming that a title-case character is a character that has both lower and upper-case variants. However, there are title-case characters that only have a lower-case variant. Use the Lt general character proprety to determine where in the case map the character should be placed instead.