[arith] Add 'shopt -s parse_empty_arith' for compatibility.

Author: akinomyoga

frontend/option_def.py

@@ -231,6 +231,7 @@ def _Init(opt_def):
 
   opt_def.Add('eval_unsafe_arith')  # recursive parsing and evaluation (ble.sh)
   opt_def.Add('parse_dynamic_arith')  # dynamic LHS
+  opt_def.Add('parse_empty_arith')  # empty arithmetic expressions
   opt_def.Add('compat_array')  # ${array} is ${array[0]}
 
   # Two strict options that from bash's shopt

osh/arith_parse.py

@@ -190,6 +190,7 @@ def __init__(self, w_parser, parse_opts):
   def Parse(self):
     # type: () -> arith_expr_t
     self.Next()  # may raise ParseError
-    if self.op_id in (Id.Arith_RParen, Id.Arith_RBrace, Id.Arith_RBracket, Id.Arith_Colon):
-      return arith_expr.Empty()
+    if self.parse_opts.parse_empty_arith():
+      if self.op_id in (Id.Arith_RParen, Id.Arith_RBrace, Id.Arith_RBracket, Id.Arith_Colon):
+        return arith_expr.Empty()
     return self.ParseUntil(0)

spec/arith.test.sh

@@ -600,6 +600,7 @@ a=1
 
 #### Empty expression: $(())
 case $SH in (dash) exit 1;; esac
+shopt -s parse_empty_arith 2>/dev/null
 echo 1:$(())
 echo 2:$(( ))
 ## STDOUT:
@@ -613,6 +614,7 @@ echo 2:$(( ))
 # dash/mksh doesn't support $[]
 # oil has different meaning for $[]
 case ${SH##*/} in (dash|mksh|osh) exit 1;; esac
+shopt -s parse_empty_arith 2>/dev/null
 echo 3:$[]
 echo 4:$[ ]
 ## STDOUT:
@@ -624,6 +626,7 @@ echo 4:$[ ]
 
 #### Empty expression: (())
 case $SH in (dash) exit 1;; esac
+shopt -s parse_empty_arith 2>/dev/null
 (()) && echo unexpected || echo OK
 (( )) && echo unexpected || echo OK
 ! (()) && echo OK || echo unexpected
@@ -643,6 +646,7 @@ case $SH in
 (mksh|zsh) exit 1;; # mksh/zsh does not support.
 (bash)     exit 1;; # bash supports this from bash-5.0.
 esac
+shopt -s parse_empty_arith 2>/dev/null
 
 arr=(foo bar)
 echo "${arr[ ]}"
@@ -655,8 +659,10 @@ foo
 #### Empty expression: ${var::}
 case $SH in
 (dash) exit 1;; # dash does not support arrays.
-(dash) exit 1;; # zsh does not support empty offset/length.
+(zsh)  exit 1;; # zsh does not support empty offset/length.
 esac
+shopt -s parse_empty_arith 2>/dev/null
+
 var=abcd
 echo "1:[${var::1}]"
 echo "2:[${var: :1}]"
@@ -681,6 +687,7 @@ case $SH in
 (mksh) exit 1;; # mksh does not support ${array[@]:offset:length}
 (zsh)  exit 1;; # zsh does not support empty offset/length
 esac
+shopt -s parse_empty_arith 2>/dev/null
 array=(1 2 3)
 argv.py "${array[@]::1}"
 argv.py "${array[@]: :1}"