${VAR:4@Q} parameter modifiers don't compose

[d+bug-bash]

Variable transforms such as ${VAR@Q} do not compose with substrings 
expansions, eg ${VAR:1:4}.

For example, I expected ${VAR:4@Q} to quote ${VAR:4}, but instead:

   -bash: VAR: 4@Q: value too great for base (error token is "4@Q")

This is because parameter value modifiers do not compose; generally
a single value modifier is supported per expansion. The 
implementation's common parameter processing code assumes
the modifier's handler requires the full text up to the next
closing '}'.

  parameter_brace_expand()
  {
      ...
      if (c == ':') want_substring=1;
      else if (c == '/') want_patsub=1;
      else if (c == '^' or ',' or '~') want_casemode=1;
      ...
      /* Get the rest of the stuff inside the braces. */
      if (c && c != RBRACE)
        {
          value = extract_dollar_brace_string(string,
              &sindex, quoted, SX_POSIXEXP | SX_WORD);
        }
      ...
      if (want_substring)
        { parameter_brace_substring(,value,); return; }
      else if (want_patsub)
        { parameter_brace_patsub(,value,); return; }
      else if (want_casemod)
        { parameter_brace_casemod(,value,); return; }
      switch (c)
        {
        case '@':
          parameter_brace_transform(,value,); return;
        }
  }

I have been thinking of a general extension that allows composition
of parameter value modifiers without changing current parsing.
It looks like this:

    ${VAR{:4}{@Q}^}

The intuitive rule here is that if a normally-terminal modifier
is placed in braces, then it may precede another or a subsequent
final unbraced modifier. Braced operands are constrained to the
next closing brace without ambiguity.

Thoughts?