Fix the evaluation of "${empty:-}".

Bug reported/reproduced by lheckemann in
ec7ca9d.

We treat it like "${empty:-''}", i.e. by inserting a dummy
SingleQuotedPart.

This is analogous to how

empty=

is equivalent to

empty=''

All spec tests pass.

core/word.py

@@ -408,8 +408,9 @@ def LooksLikeAssignment(w):

  rhs = ast.CompoundWord()
  if len(w.parts) == 1:
    # NOTE: This is necesssary so that EmptyUnquoted elision isn't
    # applied.  EMPTY= is like EMPTY=''.
    # This fake SingleQuotedPart is necesssary so that EmptyUnquoted elision
    # isn't applied.  EMPTY= is like EMPTY=''.
    # TODO: This part doesn't have spids, so it might break some invariants.
    rhs.parts.append(ast.SingleQuotedPart())
  else:
    for p in w.parts[1:]:

core/word_eval.py

@@ -991,7 +991,7 @@ def EvalWordToAny(self, word, glob_escape=False):
      return runtime.StrArray(strs)

    part_vals = self._EvalParts(word)
    #log('part_vals %s', part_vals)
    #log('EvalWordToAny part_vals %s', part_vals)

    # Instead of splitting, do a trivial transformation to frag array.
    # Example:

osh/word_parse.py

@@ -23,6 +23,7 @@
word_e = ast.word_e

p_die = util.p_die
log = util.log

# Substitutions can be nested, but which inner subs are allowed depends on the
# outer sub.  See _ReadLeftParts vs. _ReadDoubleQuotedLeftParts.
@@ -140,8 +141,18 @@ def _ReadVarOpArg(self, arg_lex_mode, eof_type=Id.Undefined_Tok,
    # valid, even when unquoted.
    self._Next(arg_lex_mode)
    self._Peek()
    return self._ReadCompoundWord(

    w = self._ReadCompoundWord(
        lex_mode=arg_lex_mode, eof_type=eof_type, empty_ok=empty_ok)
    # This is for "${s:-}", ${s/a//}, etc.  It is analogous to
    # LooksLikeAssignment where we turn x= into x=''.  It has the same
    # potential problem of not having spids.
    #
    # NOTE: empty_ok is False only for the PatSub pattern, which means we'll
    # return a CompoundWord(parts=[]), which fails later.
    if not w.parts and empty_ok:
      w.parts.append(ast.SingleQuotedPart())
    return w

  def _ReadSliceArg(self):
    """Read an arithmetic expression for either part of ${a : i+1 : i+2}."""

spec/var-sub-quote.test.sh

@@ -15,8 +15,8 @@
# makes sense I guess.  Vim's syntax highlighting is throwing me off.

### :- with empty alternative
foo=
argv.py "${foo:-}"
empty=
argv.py "${empty:-}"
# stdout: ['']

### :-