It displays 17, which is what Perl and apparently just about
everybody else used to do. I'm not convinced there was ever a
bug here as no standard I can find says you have to display 3
times as many digits as are necessary to provide a non-lossy
round trip from double to character and back.
TODO: {
local $::TODO = 'Extended precision %g formatting' if $^O eq 'cygwin'
or
+ $^O eq 'VMS'
+ or
($^O eq 'MSWin32' and
$Config{cc} eq 'cl' and
$Config{ccversion} =~ /^(\d+)/ and