[word_eval] Fix bug evaluating ${array[@]::0}.

Also add spec test for ${array[@]::}.

Addresses an issue in #653.

osh/word_eval.py

@@ -259,12 +259,12 @@ def _PerformSlice(val,  # type: value_t
       strs = []  # type: List[str]
       count = 0
       while i < n:
+        if has_length and count == length:  # length could be 0
+          break
         s = orig[i]
-        if s is not None:  # Unset elements don't count towards the length.
+        if s is not None:  # Unset elements don't count towards the length
           strs.append(s)
           count += 1
-          if has_length and count == length:
-            break
         i += 1
 
       val = value.MaybeStrArray(strs)

spec/var-op-slice.test.sh

@@ -309,3 +309,22 @@ echo ${result//"$0"/'SHELL'}
 SHELL
 ## END
 ## N-I mksh stdout-json: "\n"
+
+#### ${array[@]::0}
+array=(1 2 3)
+argv.py ${array[@]::0}
+## STDOUT:
+[]
+## END
+## N-I mksh/zsh status: 1
+## N-I mksh/zsh stdout-json: ""
+
+#### ${array[@]::}
+array=(1 2 3)
+argv.py ${array[@]::}
+## STDOUT:
+[]
+## END
+## N-I mksh/zsh status: 1
+## N-I mksh/zsh status: 1
+## N-I mksh/zsh stdout-json: ""

test/spec.sh

@@ -560,7 +560,7 @@ var-op-patsub() {
 
 var-op-slice() {
   # dash doesn't support any of these operations
-  sh-spec spec/var-op-slice.test.sh --osh-failures-allowed 0 \
+  sh-spec spec/var-op-slice.test.sh --osh-failures-allowed 1 \
     $BASH $MKSH $ZSH $OSH_LIST "$@"
 }