[bug #60802] Make .substring handle an empty result

[anonymous]

URL:
  <https://savannah.gnu.org/bugs/?60802>

                 Summary: Make .substring handle an empty result
                 Project: GNU troff
            Submitted by: None
            Submitted on: Sun 20 Jun 2021 03:01:26 AM UTC
                Category: Core
                Severity: 3 - Normal
              Item Group: Incorrect behaviour
                  Status: None
                 Privacy: Public
             Assigned to: None
             Open/Closed: Open
         Discussion Lock: Any
         Planned Release: None

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Details:

I'm trying to use .substring to remove the first character of a string,
leaving behind whatever follows, even if that is nothing (i.e. the string had
only one character).

  .substring str 1

or

  .length n2 "\*[str]
  .nr n2 -1
  .substring str 1 \n[n2]

(if n2 is omitted, it appears to default to length-1 as you might expect).

The above works as long as the string originally had at least two characters. 
If it only has one, so the desired result is the empty string, it does not
remove anything, leaving behind the 0th character and emitting a warning (with
-w all).

This seems like a corner-case bug.

At first glance, it seems like it would be safe to fix this because the result
is currently always wrong in this case, i.e. n1 equals the length of the
string (so the result should be empty).




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