Is wchar_t needed for unicode support?

No.

Technically, no. Unicode is a standard that defines code points and it does not require a particular encoding.

So, you could use unicode with the UTF-8 encoding and then everything would fit in a one or a short sequence of char objects and it would even still be null-terminated.

The problem with UTF-8 and UTF-16 is that s[i] is not necessarily a character any more, it might be just a piece of one, whereas with sufficiently wide characters you can preserve the abstraction that s[i] is a single character, tho it does not make strings fixed-length under various transformations.

32-bit integers are at least wide enough to solve the code point problem but they still don't handle corner cases, e.g., upcasing something can change the number of characters.

So it turns out that the x[i] problem is not completely solved even by char32_t, and those other encodings make poor file formats.

Your implied point, then, is quite valid: wchar_t is a failure, partly because Windows made it only 16 bits, and partly because it didn't solve every problem and was horribly incompatible with the byte stream abstraction.

You absolutely do not need wchar_t to support Unicode in the software, in fact using wchar_t makes it even harder because you do not know if "wide string" is UTF-16 or UTF-32 -- it depends on OS: under windows utf-16 all others utf-32.

However, utf-8 allows you to write Unicode enabled software easily(*)

See: https://stackoverflow.com/questions/1049947/should-utf-16-be-considered-harmful

(*) Note: under Windows you still have to use wchar_t because it does not support utf-8 locales so for unicode enabled windows programming you have to use wchar based API.

wchar_t is absolutely NOT required for Unicode. UTF-8 for example, maintains backward compatibility with ASCII and uses plain 8-bit char. wchar_t mostly yields support for so-called multi-byte characters, or basically any character set that's encoded using more than the sizeof(char).

wchar_t is not required. It's not even guaranteed to have a specific encoding. The point is to provide a data type that represents the wide characters native to your system, similar to char representing native characters. On Windows, for example, you can use wchar_t to access the wide character Win32 API functions.

Be careful, wchar_t is often 16bits which is not enough to store all unicode characters and is a bad choice ofr data in UTF_8 for instance

Because you can't accomplish the same thing with char:

The Absolute Minimum Every Software Developer Absolutely, Positively Must Know About Unicode and Character Sets (No Excuses!)

char is generally a single byte. (sizeof(char) must be equal to 1).

wchar_t was added to the language specifically to suppose multibyte characters.