As discovered in the unrelated #19494 (comment), "# E:" lines (etc) in test files are ignored at the beginning of lines. Having looked at the code, I've discovered this is because it desires the # to be preceded by a space, which is not present when the # is at the beginning of a line. This is bad because attempted error lines should not be silently ignored. (Although it is unlikely for someone to try to E an empty line.)

My PR #19546 will fix this.

Easy repro:

[case testErrorCodeOnEmptyLine]
# E: This isn't a real message, and thus this test could never pass, although somehow it currently does :)