This fixes erroneous syntax errors in parameter expansions such as

${var:-wor)d} or ${var+w(ord}. The parentheses now correctly lose
their normal grammatical meaning within the braces. Fix by Eric
Scrivner (@etscrivner) from July 2018 backported from ksh2020.

This fix complies with POSIX:
https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_06_02

src/cmd/ksh93/sh/lex.c: sh_lex():
- Set the ST_QUOTE state when analysing a modifier with parameter
  expansions using operators ':', '-', '+', '='. This state causes
  subsequent characters (including parentheses) to be considered
  quoted, suppressing their normal grammatical meaning.

src/cmd/ksh93/sh/macro.c: varsub():
- Same for skipping the expansion.

Fixes: https://github.com/ksh93/ksh/issues/126
Prior discussion: https://github.com/att/ast/issues/475

NEWS

@@ -3,6 +3,12 @@ For full details, see the git log at: https://github.com/ksh93/ksh
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2020-09-05:
+
+- Fixed erroneous syntax errors in parameter expansions such as ${var:-wor)d}
+  or ${var+w(ord}. The parentheses now correctly lose their normal grammatical
+  meaning within the braces. Fix by Eric Scrivner backported from ksh2020.
+
 2020-09-04:
 
 - Fixed a bug that caused a syntax error to be thrown if the special parameter

src/cmd/ksh93/include/version.h

@@ -17,4 +17,4 @@
 *                  David Korn <dgk@research.att.com>                   *
 *                                                                      *
 ***********************************************************************/
-#define SH_RELEASE	"93u+m 2020-09-04"
+#define SH_RELEASE	"93u+m 2020-09-05"

src/cmd/ksh93/sh/lex.c

@@ -1018,6 +1018,11 @@ int sh_lex(Lex_t* lp)
 				if(lp->kiafile)
 					refvar(lp,1);
 #endif /* SHOPT_KIA */
+				if(c == ':' || c == '-' || c == '+' || c == '=')
+				{
+					mode = ST_QUOTE;
+					continue;
+				}
 				if(c!=':' && fcgetc(n)>0)
 				{
 					if(n!=c)

src/cmd/ksh93/sh/macro.c

@@ -1628,7 +1628,8 @@ retry1:
 			}
 			else
 			{
-				sh_lexskip(lp,RBRACE,0,(!newops&&mp->quote)?ST_QUOTE:ST_NESTED);
+				int state = (!newops && mp->quote || c=='-' || c=='+' || c=='=') ? ST_QUOTE : ST_NESTED;
+				sh_lexskip(lp, RBRACE, 0, state);
 				stkseek(stkp,offset);
 			}
 			argp=stkptr(stkp,offset);

src/cmd/ksh93/tests/variables.sh

@@ -1040,5 +1040,39 @@ wait
 # ${.sh.pid} should be the same as $$ in the parent shell
 [[ $$ == ${.sh.pid} ]] || err_exit "\${.sh.pid} and \$$ differ in the parent shell (expected $$, got ${.sh.pid})"
 
+# ======
+# Parentheses after the '-', '+', and '=' expansion operators were causing syntax errors.
+# Check both the unset variable case and the set variable case for each set of symbols.
+
+unset -v foo
+for op in - :- = :=
+do	for word in '(word)' 'w(or)d' '(wor)d' 'w(ord)' 'w(ord' 'wor)d'
+	do	exp=$(set +x; eval "echo \${foo${op}${word}}" 2>&1)
+		if	[[ $exp != "$word" ]]
+		then	err_exit "\${foo${op}${word}} when foo is not set: expected \"$word\", got \"$exp\""
+	        fi
+	done
+done
+
+foo=some_value
+for op in - :- = :=
+do	for word in '(word)' 'w(or)d' '(wor)d' 'w(ord)' 'w(ord' 'wor)d'
+	do	exp=$(set +x; eval "echo \${foo${op}${word}}" 2>&1)
+		if	[[ $exp != "$foo" ]]
+		then	err_exit "\${foo${op}${word}} when foo is set: expected \"$foo\", got \"$exp\""
+		fi
+	done
+done
+
+unset -v foo
+for op in + :+
+do	for word in '(word)' 'w(or)d' '(wor)d' 'w(ord)' 'w(ord' 'wor)d'
+	do	exp=$(set +x; eval "echo \${foo${op}${word}}" 2>&1)
+		if	[[ $exp != "" ]]
+		then	err_exit "\${foo${op}${word}} when foo is not set: expected null, got \"$exp\""
+		fi
+	done
+done
+
 # ======
 exit $((Errors<125?Errors:125))