doc(styleguide,etc): describe empty-array nounset in Bash <= 4.3

Author: akinomyoga

doc/api-and-naming.md

@@ -172,7 +172,8 @@ The exit status is implementation-defined.
 
 - The `_comp_compgen -- COMPGEN_ARGS` returns whether there is at least one
   completion.  This is useful when one wants to reuse the array content with
-  `"${tmp[@]}"` avoiding `nounset` error.
+  `"${tmp[@]}"` avoiding `nounset` error in Bash 4.2 and 4.3 for an empty array
+  `tmp`.
 - Some use other rules for the exit status. E.g., `help` and `usage` return
   whether there were options *before* filtering by cur. This is used for
   `_comp_compgen_help || _comp_compgen_usage`.
@@ -206,8 +207,8 @@ in `_comp_compgen_mygen1`.
 _comp_compgen_mygen1()
 {
     local -a arr=(1 2 3)
-    _comp_compgen -av arr -- -W '4 5 6'
-    _comp_compgen_set "${arr[@]/#p}"
+    _comp_compgen -av arr -- -W '4 5 6' &&
+      _comp_compgen_set "${arr[@]/#p}"
 }
 
 _comp_compgen -v arr mygen1 # fails to get the result in array `arr`
@@ -222,8 +223,8 @@ assigning the final result.
 _comp_compgen_mygen1()
 {
     local -a arr=(1 2 3)
-    _comp_compgen -av arr -- -W '4 5 6'
-    _comp_compgen -U arr set "${arr[@]/#p}"
+    _comp_compgen -av arr -- -W '4 5 6' &&
+      _comp_compgen -U arr set "${arr[@]/#p}"
 }
 ```
 

doc/styleguide.md

@@ -111,6 +111,16 @@ E.g. write `${foo[bar]}`, not `${foo[$bar]}`, and similarly `${foo[bar+1]}`
 vs `${foo[((bar+1))]}` or `${foo[$((bar+1))]}`, `${foo[--i]}` vs
 `${foo[((--i))]}`.
 
+## `((${#array[@]})) && cmd "${array[@]}"`
+
+When one uses the array expansion of the form `"${array[@]}"` or `${array[*]}`,
+please make sure that the `array` is not empty.  This is because of a strange
+behavior of Bash <= 4.3, where `${array[@]}` and `${array[*]}` fail for the
+`nounset` error when `array` is an empty array.  It can be explicitly tested
+with `((${#array[@]}))`, or the exit status of `_comp_compgen` that assigned
+values to the array can be used (when the generator is designed to return the
+appropriate exit status).
+
 ## Loop variable names
 
 Use `i`, `j`, `k` for loop-local indices; `n` and `m` for lengths; some other