tr: echo 1|tr . \\ lacks warning

[oech3]

$ echo 1|tr . \\
1
$ echo 1|/home/a/bin/gnu9.11/tr . \\
tr: warning: an unescaped backslash at end of string is not portable
1

2>/dev/full should exit 1.

[oech3]

and

$ echo .|/home/a/bin/gnu9.11/tr . \\ 2>/dev/full
\ # exit 1