wcwidth: avoid test failure on AIX 7.2

[Bruno Haible]

On AIX 7.2, the wcwidth test fails:

../../gltests/test-wcwidth.c:72: assertion 'wcwidth (0x200B) == 0' failed

While it is better if wcwidth(0x200B) is zero (because it makes wcswidth
work better), it can be justified that wcwidth(0x200B) is -1:
  - In Unicode, the character 0x200B is of category Cf (= Format).
  - Therefore AIX does not include this character among the printable or
    even the graphic characters (iswprint(0x200B) = iswgraph(0x200B) = 0).
  - POSIX [1] says that if a character is not printable, wcwidth returns
    -1 for it.

[1] https://pubs.opengroup.org/onlinepubs/9699919799/functions/wcwidth.html


2019-12-12  Bruno Haible  <address@hidden>

        wcwidth: Avoid test failure on AIX 7.2.
        * tests/test-wcwidth.c (main): Don't fail if wcwidth(0x200B) is
        negative.
        * doc/posix-functions/wcwidth.texi: Mention the AIX issue.

diff --git a/doc/posix-functions/wcwidth.texi b/doc/posix-functions/wcwidth.texi
index 0acc1c4..a3afe9b 100644
--- a/doc/posix-functions/wcwidth.texi
+++ b/doc/posix-functions/wcwidth.texi
@@ -29,4 +29,8 @@ Portability problems not fixed by Gnulib:
 @item
 On AIX and Windows platforms, @code{wchar_t} is a 16-bit type and therefore 
cannot
 accommodate all Unicode characters.
+@item
+This function treats zero-width spaces like control characters on some
+platforms:
+AIX 7.2.
 @end itemize
diff --git a/tests/test-wcwidth.c b/tests/test-wcwidth.c
index 8e9cea3..ecb50cd 100644
--- a/tests/test-wcwidth.c
+++ b/tests/test-wcwidth.c
@@ -69,7 +69,11 @@ main ()
 #endif
 
       /* Test width of some zero width characters.  */
-      ASSERT (wcwidth (0x200B) == 0);
+      /* While it is desirable that U+200B, U+200C, U+200D have width 0,
+         because this makes wcswidth work better on strings that contain these
+         characters, it is acceptable if an implementation treats these
+         characters like control characters.  */
+      ASSERT (wcwidth (0x200B) <= 0);
       ASSERT (wcwidth (0xFEFF) <= 0);
 
       /* Test width of some math symbols.