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

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))