Fix remaining bug in ${var:-'{}'} (re: d087b03)

The following problems remained:

$ var=x; echo ${var:-'{}'}
x}
$ var=; echo ${var:+'{}'}
}

src/cmd/ksh93/sh/macro.c: varsub():
- Use the new ST_MOD1 state table to skip over ${var-'foo'}, etc.
  instead of ST_QUOTE. In ST_MOD1 the ' is categorised as S_LIT
  which causes the single quotes to be skipped over correctly.
  See d087b03 for more info.

src/cmd/ksh93/tests/quoting2.sh:
- Add tests for this remaining bug.
- Make the new test xtrace-proof.

Resolves: #290 (again)

NEWS

@@ -3,6 +3,11 @@ For full details, see the git log at: https://github.com/ksh93/ksh
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2021-05-03:
+
+- Completed the 2021-04-30 fix for ${var<OP>'{}'} where <OP> is '-', '+',
+  ':-' or ':+' by fixing a bug that caused an extra '}' to be output.
+
 2021-04-30:
 
 - The emacs 'ESC .' (M-.) and vi '_' commands now take shell quoting into

src/cmd/ksh93/include/version.h

@@ -21,7 +21,7 @@
 
 #define SH_RELEASE_FORK	"93u+m"		/* only change if you develop a new ksh93 fork */
 #define SH_RELEASE_SVER	"1.0.0-alpha"	/* semantic version number: https://semver.org */
-#define SH_RELEASE_DATE	"2021-04-30"	/* must be in this format for $((.sh.version)) */
+#define SH_RELEASE_DATE	"2021-05-03"	/* must be in this format for $((.sh.version)) */
 #define SH_RELEASE_CPYR	"(c) 2020-2021 Contributors to ksh " SH_RELEASE_FORK
 
 /* Scripts sometimes field-split ${.sh.version}, so don't change amount of whitespace. */

src/cmd/ksh93/sh/macro.c

@@ -1659,7 +1659,7 @@ static int varsub(Mac_t *mp)
 			}
 			else
 			{
-				sh_lexskip(lp, RBRACE, 0, sh_lexstates[ST_BRACE][c]==S_MOD1 ? ST_QUOTE : ST_NESTED);
+				sh_lexskip(lp, RBRACE, 0, sh_lexstates[ST_BRACE][c]==S_MOD1 ? ST_MOD1 : ST_NESTED);
 				stkseek(stkp,offset);
 			}
 			argp=stkptr(stkp,offset);

src/cmd/ksh93/tests/quoting2.sh

@@ -254,14 +254,30 @@ actual=$(printf %q $'1\x[11]1')
 # https://github.com/ksh93/ksh/issues/290
 var=dummy
 exp='{}'
-got=$(eval 'echo ${var:+'\''{}'\''}' 2>&1)
+got=$(set +x; eval 'echo ${var:+'\''{}'\''}' 2>&1)
 [[ $got == "$exp" ]] || err_exit "Single quotes misparsed in expansion operator string (1)" \
 	"(expected $(printf %q "$exp"), got $(printf %q "$got"))"
 unset var
 exp='}'
-got=$(eval 'echo ${var:-'\''}'\''}' 2>&1)
+got=$(set +x; eval 'echo ${var:-'\''}'\''}' 2>&1)
 [[ $got == "$exp" ]] || err_exit "Single quotes misparsed in expansion operator string (2)" \
 	"(expected $(printf %q "$exp"), got $(printf %q "$got"))"
+exp='x'
+got=$(var=x; set +x; eval 'echo ${var:-'\''{}'\''}' 2>&1)
+[[ $got == "$exp" ]] || err_exit "Single quotes misparsed in expansion operator string (3)" \
+	"(expected $(printf %q "$exp"), got $(printf %q "$got"))"
+exp=''
+got=$(var=; set +x; eval 'echo ${var:+'\''{}'\''}' 2>&1)
+[[ $got == "$exp" ]] || err_exit "Single quotes misparsed in expansion operator string (4)" \
+	"(expected $(printf %q "$exp"), got $(printf %q "$got"))"
+exp='{}'
+got=$(unset var; set +x; eval 'echo ${var-'\''{}'\''}' 2>&1)
+[[ $got == "$exp" ]] || err_exit "Single quotes misparsed in expansion operator string (5)" \
+	"(expected $(printf %q "$exp"), got $(printf %q "$got"))"
+exp=''
+got=$(unset var; set +x; eval 'echo ${var+'\''{}'\''}' 2>&1)
+[[ $got == "$exp" ]] || err_exit "Single quotes misparsed in expansion operator string (6)" \
+	"(expected $(printf %q "$exp"), got $(printf %q "$got"))"
 
 # ======
 exit $((Errors<125?Errors:125))