Add a failing test for dynamic parsing of assignments.

This came up in the bash-completion project, with 'local $2 $3 $4'.  But
wrapping it in 'eval' is an easy workaround.

osh/cmd_parse.py

@@ -203,8 +203,14 @@ def _AppendMoreEnv(preparsed_list, more_env):
 
 
 def _MakeAssignment(parse_ctx, assign_kw, suffix_words):
-  """Create an ast.Assignment node from a keyword and a list of words."""
+  """Create an ast.Assignment node from a keyword and a list of words.
 
+  NOTE: We don't allow dynamic assignments like:
+
+  local $1
+
+  This can be replaced with eval 'local $1'
+  """
   # First parse flags, e.g. -r -x -a -A.  None of the flags have arguments.
   flags = []
   n = len(suffix_words)

spec/assign.test.sh

@@ -413,3 +413,20 @@ echo $a $b $c
 ## OK mksh stdout-json: ""
 ## OK mksh status: 1
 
+#### dynamic local variables
+f() {
+  local "$1"  # Only x is assigned here
+  echo [$x]
+  echo [$a]
+
+  local $1  # x and a are assigned here
+  echo [$x]
+  echo [$a]
+}
+f 'x=y a=b'
+## STDOUT:
+[y a=b]
+[]
+[y]
+[b]
+## END

test/spec.sh

@@ -251,7 +251,7 @@ word-eval() {
 }
 
 assign() {
-  sh-spec spec/assign.test.sh --osh-failures-allowed 2 \
+  sh-spec spec/assign.test.sh --osh-failures-allowed 3 \
     ${REF_SHELLS[@]} $OSH_LIST "$@" 
 }