[osh] Allow (( )) and $(( ))

They implicit evaluate to zero, and in the (( )) case, the exit code
becomes 1.

Shells widely agree on this, and Samuel found a use by Nix

After preprocessing

    if (( @isClang )); then

It became

    if (( )); then

osh/word_parse.py

@@ -1479,14 +1479,15 @@ def _ReadArithSub(self):
         # $((echo * foo))  # looks like multiplication
         # $((echo / foo))  # looks like division
 
-        self._SetNext(lex_mode_e.Arith)
-        anode = self._ReadArithExpr(Id.Arith_RParen)
+        anode = arith_expr.EmptyZero
+        self._SetNextNonSpace()
+
+        if self.token_type != Id.Arith_RParen:
+            anode = self._ReadArithExpr(Id.Arith_RParen)
 
-        # TODO: This could be DQ or Arith too
         self._SetNext(lex_mode_e.ShCommand)
 
-        # PROBLEM: $(echo $(( 1 + 2 )) )
-        # Two right parens break the Id.Eof_RParen scheme
+        # Ensure we get closing )
         self._GetToken()
         if self.token_type != Id.Right_DollarDParen:
             p_die('Expected second ) to end arith sub', self.cur_token)
@@ -1501,24 +1502,25 @@ def ReadDParen(self):
         We're using the word parser because it's very similar to _ReadArithExpr
         above.
 
-        This also returns the terminating `Op_DRightParen` token for use as location
-        tracking.
+        This also returns the terminating Id.Op_DRightParen token for location
+        info.
         """
-        # The second one needs to be disambiguated in stuff like stuff like:
-        # TODO: Be consistent with ReadForExpression below and use lex_mode_e.Arith?
-        # Then you can get rid of this.
+        anode = arith_expr.EmptyZero  # (( ))
+
         self.lexer.PushHint(Id.Op_RParen, Id.Op_DRightParen)
 
-        self._SetNext(lex_mode_e.Arith)
-        anode = self._ReadArithExpr(Id.Arith_RParen)
+        self._SetNextNonSpace()
+        self._GetToken()
+        if self.token_type != Id.Arith_RParen:
+            anode = self._ReadArithExpr(Id.Arith_RParen)
 
         self._SetNext(lex_mode_e.ShCommand)
 
-        # PROBLEM: $(echo $(( 1 + 2 )) )
+        # Ensure we get the second )
         self._GetToken()
         right = self.cur_token
-        if self.token_type != Id.Op_DRightParen:
-            p_die('Expected second ) to end arith statement', self.cur_token)
+        if right.id != Id.Op_DRightParen:
+            p_die('Expected second ) to end arith statement', right)
 
         self._SetNext(lex_mode_e.ShCommand)
 

spec/arith-context.test.sh

@@ -1,5 +1,5 @@
 ## compare_shells: bash mksh zsh
-## oils_failures_allowed: 2
+## oils_failures_allowed: 1
 
 # Test arithmetic expressions in all their different contexts.
 

test/parse-errors.sh

@@ -117,6 +117,42 @@ test-word-parse() {
   _osh-parse-error '${x:'
 }
 
+test-dparen() {
+  # (( ))
+
+  _osh-should-parse '(())'
+  _osh-should-parse '(( ))'
+  _osh-parse-error '(( )'
+  _osh-parse-error '(( )x'
+  #_osh-should-parse '$(echo $(( 1 + 2 )) )'
+
+  # Hard case
+  _osh-should-parse '$(echo $(( 1 + 2 )))'
+  _osh-should-parse '$( (()))'
+
+  # More
+  _osh-parse-error '(( 1 + 2 /'
+  _osh-parse-error '(( 1 + 2 )/'
+  _osh-parse-error '(( 1'
+  _osh-parse-error '(('
+}
+
+test-arith-sub() {
+  # $(( ))
+
+  _osh-should-parse 'echo $(( ))'
+  _osh-should-parse 'echo $(())'
+  _osh-parse-error 'echo $(()x'
+
+  _osh-parse-error 'echo $(()'
+
+  _osh-parse-error 'echo $(( 1 + 2 ;'
+  _osh-parse-error 'echo $(( 1 + 2 );'
+  _osh-parse-error 'echo $(( '
+  _osh-parse-error 'echo $(( 1'
+}
+
+
 test-array-literal() {
   # Array literal with invalid TokenWord.
   _osh-parse-error 'a=(1 & 2)'
@@ -126,12 +162,6 @@ test-array-literal() {
 }
 
 test-arith-context() {
-  # $(( ))
-  _osh-parse-error 'echo $(( 1 + 2 ;'
-  _osh-parse-error 'echo $(( 1 + 2 );'
-  _osh-parse-error 'echo $(( '
-  _osh-parse-error 'echo $(( 1'
-
   # Disable Oil stuff for osh_{parse,eval}.asan
   if false; then
     # Non-standard arith sub $[1 + 2]
@@ -144,12 +174,6 @@ test-arith-context() {
     _osh-parse-error 'echo $['
   fi
 
-  # (( ))
-  _osh-parse-error '(( 1 + 2 /'
-  _osh-parse-error '(( 1 + 2 )/'
-  _osh-parse-error '(( 1'
-  _osh-parse-error '(('
-
   # Should be an error
   _osh-parse-error 'a[x+]=1'
 
@@ -751,7 +775,6 @@ esac'
     echo bash=$?
     set -o errexit
   done
-
 }
 
 all() {