Fix 'export foo+=bar', etc.

Example reproducers:

  $ typeset foo+=bar
  -ksh: typeset: foo+: invalid variable name
  $ export foo+=bar
  -ksh: export: foo+: is not an identifier

Expected result: 'bar' is appended to the value of foo and (in the
'export' case) then exported. This works on mksh and bash.

Oddly enough, the following does work:

  $ unset foo
  $ foo[23]=bar
  $ typeset foo[23]+=baz
  $ typeset -p foo
  typeset -x -a foo=([23]=barbaz)

The fact that it works *with* an array subscript strongly suggests
that it is supposed to work without one. Indeed, testing historical
ksh versions shows that this used to work but stopped working in
version 93q+ 2005-05-22.

The simple() function in parse.c, where it isolates a variable name
from the rest of the assignment, has explicit code for recognising
array subscripts preceding both '=' and '+='. But the same is simply
not implemented for an assignment with a simple variable name.

src/cmd/ksh93/sh/parse.c: simple():
- Add that missing implementation: decrease 'last' (pointer to the
  character following the last character of the variable name) if
  '+' is found before '='.

NEWS

@@ -2,6 +2,12 @@ This documents significant changes in the 1.0 branch of ksh 93u+m.
 For full details, see the git log at: https://github.com/ksh93/ksh/tree/1.0
 Uppercase BUG_* IDs are shell bug IDs as used by the Modernish shell library.
 
+2024-01-15:
+
+- Fixed a bug, introduced in ksh93q+ 2005-05-22, that stopped an append
+  assignment from working together with a declaration command. For example,
+  'typeset var+=value' or 'export var+=value' now again work as expected.
+
 2024-01-14:
 
 - Fixed a regression introduced on 2022-11-01 that caused ksh to enter

src/cmd/ksh93/include/version.h

@@ -18,7 +18,7 @@
 
 #define SH_RELEASE_FORK	"93u+m"		/* only change if you develop a new ksh93 fork */
 #define SH_RELEASE_SVER	"1.0.9-beta"	/* semantic version number: https://semver.org */
-#define SH_RELEASE_DATE	"2024-01-14"	/* must be in this format for $((.sh.version)) */
+#define SH_RELEASE_DATE	"2024-01-15"	/* must be in this format for $((.sh.version)) */
 #define SH_RELEASE_CPYR	"(c) 2020-2024 Contributors to ksh " SH_RELEASE_FORK
 
 /* Scripts sometimes field-split ${.sh.version}, so don't change amount of whitespace. */

src/cmd/ksh93/sh/parse.c

@@ -1483,6 +1483,8 @@ static Shnode_t *simple(Lex_t *lexp,int flag, struct ionod *io)
 					last = strchr(argp->argval,'=');
 					if(last && (last[-1]==']'|| (last[-1]=='+' && last[-2]==']')) && (cp=strchr(argp->argval,'[')) && (cp < last) && cp[-1]!='.')
 						last = cp;
+					else if(last && last[-1]=='+')
+						last--;
 					stkseek(sh.stk,ARGVAL);
 					sfwrite(sh.stk,argp->argval,last-argp->argval);
 					ap=(struct argnod*)stkfreeze(sh.stk,1);

src/cmd/ksh93/tests/append.sh

@@ -2,7 +2,7 @@
 #                                                                      #
 #               This software is part of the ast package               #
 #          Copyright (c) 1982-2012 AT&T Intellectual Property          #
-#          Copyright (c) 2020-2022 Contributors to ksh 93u+m           #
+#          Copyright (c) 2020-2024 Contributors to ksh 93u+m           #
 #                      and is licensed under the                       #
 #                 Eclipse Public License, Version 2.0                  #
 #                                                                      #
@@ -104,4 +104,18 @@ typeset -a arr2
 } 2> /dev/null
 [[ $(typeset -p arr2) == "$exp" ]] || err_exit 'append (b=c xxxxx) to indexed array not working'
 
+# ======
+unset foo
+exp='typeset -x foo=barbaz'
+got=$(foo=bar; export foo+=baz 2>&1 && typeset -p foo)
+[[ e=$? -eq 0 && $got == "$exp" ]] || err_exit 'declaration command does not support +=' \
+	"(expected status 0, $(printf %q "$exp");" \
+	"got status $e$( ((e>128)) && print -n /SIG && kill -l "$e"), $(printf %q "$got"))"
+exp='typeset -x -a foo=([23]=barbaz)'
+got=$(foo[23]=bar; export foo[23]+=baz 2>&1 && typeset -p foo)
+[[ e=$? -eq 0 && $got == "$exp" ]] || err_exit 'declaration command does not support +=' \
+	"(expected status 0, $(printf %q "$exp");" \
+	"got status $e$( ((e>128)) && print -n /SIG && kill -l "$e"), $(printf %q "$got"))"
+
+# ======
 exit $((Errors<125?Errors:125))