Yes. If the problem was trying to parse ASCII consistently that would be the right solution.
My response was to OPs moving the goal post to "portably parsing ASCII" in response to his suggested replacement for a C library function not being portable on non-ASCII systems, which make no sense.
I explicitly wrote "or whatever test" as a comment to the code snippet, and obviously the test was not the point of my comment.
Anyway I think most programming languages nowadays have their source encoding specified as UTF-8 or at least something ASCII-like, so ('A' <= c && c <= 'Z') is in fact what I would likely write, and using isalpha() would technically be a bug just as well.
EBCDIC famously does not have A..Z as contiguous characters, and I wouldn't describe it as a 'franken-coding' just yet - it still finds plenty of use in some places.
Unless you're dealing with mainframes, it's not like you see it everyday.
Wait, what? If C does not require A..Z to be contiguous, the distinction between 'A' and 0x41 is extremely significant to portable programs intending to parse ASCII when the native compiler encoding is whatever franken-coding doesn't have contiguous latin characters.