> Write a program which accepts a letter and display it in uppercase

That task is a lot harder than it sounds, given that casing is locale-specific. In a Turkish locale for example, I would expect a lowercase "i" to be converted to U+0130 "İ" instead of "I" like in en-US.

And then there's the whole ambiguity around the word "letter". Is a letter a single code point, or a grapheme cluster? What if someone passes in an emoji? With multiple zero-width joiners? Better make sure your dependencies are up to date so you know what's a single letter and what's too many...