Fix spurious syntax error when using ${foo[${bar}..${baz}]} (ksh93#436)

Attempting to use array subscript expansion with variables that
aren't set currently causes a spurious syntax error (in ksh93u+ and
older commits the reproducer crashes):
   $ ksh -c 'echo ${foo[${bar}..${baz}]}'  # Shouldn't print anything
   ksh: : arithmetic syntax error

src/cmd/ksh93/sh/macro.c:
- Backport a parser bugfix from ksh93v- 2012-08-24 that avoids
  setting mp->dotdot until the copyto() function's loop is
  finished.

src/cmd/ksh93/tests/arrays.sh:
- Add regression tests for this bug.

NEWS

@@ -8,6 +8,9 @@ Any uppercase BUG_* names are modernish shell bug IDs.
 - On Cygwin, ksh now executes scripts that do not have a #! path itself,
   like it does on other systems, instead of with /bin/sh.
 
+- Fixed a spurious syntax error which occurred when attempting to use array
+  subscript expansion with unset variables (i.e., ${foo[${bar}..${baz}]}).
+
 2022-01-24:
 
 - Fixed a crashing bug in history expansion that could occur when using the "&"

src/cmd/ksh93/sh/macro.c

@@ -438,6 +438,7 @@ static void copyto(register Mac_t *mp,int endch, int newquote)
 	int		ansi_c = 0;
 	int		paren = 0;
 	int		ere = 0;
+	int		dotdot = 0;
 	int		brace = 0;
 	Sfio_t		*sp = mp->sp;
 	Stk_t		*stkp = sh.stk;
@@ -843,14 +844,15 @@ static void copyto(register Mac_t *mp,int endch, int newquote)
 			{
 				sfwrite(stkp,first,c);
 				sfputc(stkp,0);
-				mp->dotdot = stktell(stkp);
+				dotdot = stktell(stkp);
 				cp = first = fcseek(c+2);
 			}
 			break;
 		}
 	}
 done:
 	mp->sp = sp;
+	mp->dotdot = dotdot;
 	mp->quote = oldquote;
 }
 

src/cmd/ksh93/tests/arrays.sh

@@ -739,5 +739,20 @@ typeset -a foo=([1]=w [2]=x) bar=(a b c)
 foo+=("${bar[@]}")
 [[ $(typeset -p foo) == 'typeset -a foo=([1]=w [2]=x [3]=a [4]=b [5]=c)' ]] || err_exit 'Appending does not work if array contains empty indexes'
 
+# ======
+# Array expansion should work without crashing or
+# causing spurious syntax errors.
+exp='b c'
+got=$("$SHELL" -c '
+       typeset -a ar=([0]=a [1]=b [2]=c)
+       integer a=1 b=2
+       print ${ar[${a}..${b}]}
+' 2>&1)
+[[ $exp == $got ]] || err_exit $'Substituting array elements with ${ar[${a}..${b}]} doesn\'t work' \
+	"(expected $(printf %q "$exp"), got $(printf %q "$got"))"
+got=$("$SHELL" -c 'test -z ${foo[${bar}..${baz}]}' 2>&1)
+[[ -z $got ]] || err_exit $'Using ${foo[${bar}..${baz}]} when the variable ${foo} isn\'t set fails in error' \
+	"(got $(printf %q "$got"))"
+
 # ======
 exit $((Errors<125?Errors:125))