[osh] Fix ${unset_variable@Q} wrongly producing ''

core/runtime.asdl

@@ -59,7 +59,7 @@ module runtime
   coerced = Int | Float | Neither
 
   # evaluation state for BracedVarSub 
-  VarSubState = (bool join_array, bool is_type_query, bool has_test_op)
+  VarSubState = (bool join_array, bool is_type_query, bool has_test_op, bool has_nullary_op)
 
   # A Cell is a wrapper for a value.
   # TODO: add location for declaration for 'assigning const' error

doc/ref/chap-word-lang.md

@@ -237,6 +237,36 @@ The pattern can also be a glob:
 
 ### op-format
 
-${x@P} evaluates x as a prompt string, e.g. the string that would be printed if
+${x@P} evaluates x as a prompt string, i.e. the string that would be printed if
 PS1=$x.
 
+---
+
+`${x@Q}` quotes the value of `x`, if necessary, so that it can be evaluated as
+a shell word.
+
+    $ x='<'
+    $ echo "value = $x, quoted = ${x@Q}."
+    value = <, quoted = '<'.
+
+    $ x=a
+    $ echo "value = $x, quoted = ${x@Q}."
+    value = a, quoted = a.
+
+In the second case, the string `a` doesn't need to be quoted.
+
+---
+
+Format operations like `@Q` generally treat **empty** variables differently
+than **unset** variables.
+
+That is, `${empty@Q}` is the string `''`, while `${unset@Q}` is an empty
+string:
+
+    $ x=''
+    $ echo "value = $x, quoted = ${x@Q}."
+    value = , quoted = ''.
+
+    $ unset -v x
+    $ echo "value = $x, quoted = ${x@Q}."
+    value = , quoted = .

doc/ref/toc-osh.md

@@ -147,7 +147,7 @@ X [Unsupported]   enable
                   op-patsub     ${x//y/z}
                   op-index      ${a[i+1}
                   op-slice      ${a[@]:0:1}
-                  op-format     ${x@P}
+                  op-format     ${x@P} ${x@Q} etc.
 ```
 
 <h2 id="mini-lang">

osh/word_eval.py

@@ -441,7 +441,8 @@ def _PerformSlice(
                         i = i + 1
 
                     if has_length:
-                        strs = bash_impl.SparseArray_GetValues(val)[i:i+length]
+                        strs = bash_impl.SparseArray_GetValues(val)[i:i +
+                                                                    length]
                     else:
                         strs = bash_impl.SparseArray_GetValues(val)[i:]
 
@@ -1043,7 +1044,9 @@ def _Nullary(self, val, op, var_name):
         op_id = op.id
         if op_id == Id.VOp0_P:
             with tagswitch(val) as case:
-                if case(value_e.Str):
+                if case(value_e.Undef):
+                    result = value.Str('')
+                elif case(value_e.Str):
                     str_val = cast(value.Str, UP_val)
                     prompt = self.prompt_ev.EvalPrompt(str_val)
                     # readline gets rid of these, so we should too.
@@ -1054,7 +1057,9 @@ def _Nullary(self, val, op, var_name):
 
         elif op_id == Id.VOp0_Q:
             with tagswitch(val) as case:
-                if case(value_e.Str):
+                if case(value_e.Undef):
+                    result = value.Str('')
+                elif case(value_e.Str):
                     str_val = cast(value.Str, UP_val)
                     result = value.Str(j8_lite.MaybeShellEncode(str_val.s))
                     # oddly, 'echo ${x@Q}' is equivalent to 'echo "${x@Q}"' in
@@ -1120,7 +1125,8 @@ def _WholeArray(self, val, part, quoted, vsub_state):
                 if self.exec_opts.strict_array():
                     e_die("Can't index string with %s" % op_str,
                           loc.WordPart(part))
-            elif case2(value_e.BashArray, value_e.SparseArray, value_e.BashAssoc):
+            elif case2(value_e.BashArray, value_e.SparseArray,
+                       value_e.BashAssoc):
                 pass  # no-op
             else:
                 # The other YSH types such as List, Dict, and Float are not
@@ -1418,6 +1424,8 @@ def _EvalBracedVarSub(self, part, part_vals, quoted):
                 if case(suffix_op_e.Nullary):
                     suffix_op_ = cast(Token, UP_op)
 
+                    vsub_state.has_nullary_op = True
+
                     # Type query ${array@a} is a STRING, not an array
                     # NOTE: ${array@Q} is ${array[0]@Q} in bash, which is different than
                     # ${array[@]@Q}
@@ -1476,7 +1484,8 @@ def _EvalBracedVarSub(self, part, part_vals, quoted):
                 raise AssertionError(part.prefix_op)
 
         else:
-            if not vsub_state.has_test_op:  # undef -> '' if no prefix op
+            # undef -> '' if no prefix op or ${x@P} up
+            if not vsub_state.has_test_op and not vsub_state.has_nullary_op:
                 val = self._EmptyStrOrError(val, part.token)
 
         quoted2 = False  # another bit for @Q
@@ -1487,7 +1496,6 @@ def _EvalBracedVarSub(self, part, part_vals, quoted):
                 if case(suffix_op_e.Nullary):
                     op = cast(Token, UP_op)
                     val, quoted2 = self._Nullary(val, op, var_name)
-
                 elif case(suffix_op_e.Unary):
                     op = cast(suffix_op.Unary, UP_op)
                     if consts.GetKind(op.op.id) == Kind.VTest:

spec/var-op-bash.test.sh

@@ -280,14 +280,6 @@ status=0
 
 status=0
 
-status=0
-## END
-## OK osh STDOUT:
-
-status=0
-''
-status=0
-
 status=0
 ## END