23562: add KSH_ZERO_SUBSCRIPT option and leave off by default

Test/C01arith.ztst

@@ -91,8 +91,13 @@
 >3.5
 >4
 
-  (( newarray[unsetvar]++ ))
-  (( newarray[unsetvar]++ ))
+  (( newarray[unsetvar] = 1 ))
+2:error using unset variable as index
+?(eval):1:  assignment to invalid subscript range
+
+  integer setvar=1
+  (( newarray[setvar]++ ))
+  (( newarray[setvar]++ ))
   print ${(t)newarray} ${#newarray} ${newarray[1]}
 0:setting array elements in math context
 >array 1 2

Test/D05array.ztst

@@ -30,12 +30,12 @@
 >..
 
   echo .$foo[0].
-0:Treat 0 like 1
->.a.
+0:Treat 0 as empty
+>..
 
   echo .$foo[0,0].
-0:Treat 0,0 like 1,1.
->.a.
+0:Treat 0,0 as empty
+>..
 
   echo .$foo[0,1].
 0:Another weird way to access the first element

Test/D06subscript.ztst

@@ -182,3 +182,56 @@
   echo X${${l##*}[-1]}X
 0:Negative index applied to substition result from empty array
 >XX
+
+  array=(one two three four)
+  print X$array[0]X
+0:Element zero is empty if KSH_ZERO_SUBSCRIPT is off.
+>XX
+
+  array[0]=fumble
+1:Can't set element zero if KSH_ZERO_SUBSCRIPT is off.
+?(eval):1: array: assignment to invalid subscript range
+
+  print X$array[(R)notfound]X
+0:(R) returns empty if not found if KSH_ZERO_SUBSCRIPT is off.
+>XX
+
+  setopt KSH_ZERO_SUBSCRIPT
+  print X$array[0]X
+0:Element zero is element one if KSH_ZERO_SUBSCRIPT is on.
+>XoneX
+
+  array[0]=fimble
+  print $array
+0:Can set element zero if KSH_ZERO_SUBSCRIPT is on.
+>fimble two three four
+
+  print X$array[(R)notfound]X
+0:(R) yuckily returns the first element on failure withe KSH_ZERO_SUBSCRIPT
+>XfimbleX
+
+  unsetopt KSH_ZERO_SUBSCRIPT
+  array[(R)notfound,(r)notfound]=(help help here come the seventies retreads)
+  print $array
+0:[(R)notfound,(r)notfound] replaces the whole array
+>help help here come the seventies retreads
+
+  string="Why, if it isn't Officer Dibble"
+  print "[${string[0]}][${string[1]}][${string[0,3]}]"
+0:String subscripts with KSH_ZERO_SUBSCRIPT unset
+>[][W][Why]
+
+  setopt KSH_ZERO_SUBSCRIPT
+  print "[${string[0]}][${string[1]}][${string[0,3]}]"
+0:String subscripts with KSH_ZERO_SUBSCRIPT set
+>[W][W][Why]
+
+  unsetopt KSH_ZERO_SUBSCRIPT
+  string[0,3]="Goodness"
+  print $string
+0:Assignment to chunk of string ignores element 0
+>Goodness, if it isn't Officer Dibble
+
+  string[0]=!
+1:Can't set only element zero of string
+?(eval):1: string: assignment to invalid subscript range

Test/D07multibyte.ztst

@@ -88,7 +88,7 @@
   s=é
   print A${s[-2]}A B${s[-1]}B C${s[0]}C D${s[1]}D E${s[2]}E
 0:Out of range subscripts with multibyte characters
->AA BéB CéC DéD EE
+>AA BéB CC DéD EE
 
   print ${a[(i)é]} ${a[(I)é]} ${a[${a[(i)é]},${a[(I)é]}]}
 0:Reverse indexing with multibyte characters

Test/E01options.ztst

@@ -521,7 +521,7 @@
 >one one[2]
 >one two three
 >one two three two
->one one two three
+>one two three
 
   fpath=(.)
   echo >foo 'echo foo loaded; foo() { echo foo run; }'