On Wed, Apr 18, 2018 at 2:31 PM, Johannes Schindelin
[off-list ref] wrote:

I suspect that the culprit is once again Cygwin's trick where illegal
characters are mapped into a private Unicode page. Cygwin (and therefore
MSYS2 runtime, and therefore the Bash used to run the test script) can use
those filenames all right, but Git cannot.

So even testing whether you could write an illegal file name via shell
script is *not* enough to determine whether the file system supports funny
characters.

I followed suit of all existing FUNNYNAMES checks:

  $ git grep -B3 'test_set_prereq FUNNYNAMES' master t/
  master:t/t3600-rm.sh-if test_have_prereq !MINGW && touch -- 'tab
   embedded' 'newline
  master:t/t3600-rm.sh-embedded' 2>/dev/null
  master:t/t3600-rm.sh-then
  master:t/t3600-rm.sh:   test_set_prereq FUNNYNAMES
  --
  master:t/t4135-apply-weird-filenames.sh-        if test_have_prereq !MINGW &&
  master:t/t4135-apply-weird-filenames.sh-                touch --
"tab   embedded.txt" '\''"quoteembedded".txt'\''
  master:t/t4135-apply-weird-filenames.sh-        then
  master:t/t4135-apply-weird-filenames.sh:
test_set_prereq FUNNYNAMES
  --
  master:t/t9903-bash-prompt.sh-
  master:t/t9903-bash-prompt.sh-if test_have_prereq !MINGW && mkdir
"$repo_with_newline" 2>/dev/null
  master:t/t9903-bash-prompt.sh-then
  master:t/t9903-bash-prompt.sh:  test_set_prereq FUNNYNAMES

How am I supposed to check this, then?

As far as I can tell from a *really* cursory glance, this is the only
affected test case. Apparently your prereq catches, somehow, on Windows:

2018-04-18T11:12:43.0459702Z     Your filesystem does not allow \ and " in filenames.
2018-04-18T11:12:43.0459823Z     skipped: complete files - C-style escapes in ls-files output (missing FUNNYNAMES_BS_DQ)