Why is "${1-"$var"}" (option 6 down below) not mentioned in POSIX?

The only reference I could find in the spec is this : > It would be desirable to include the statement "The characters from an enclosed "${" to the matching '}' shall not be affected by the double-quotes", similar to the one for "$()". However, historical practice in the System V shell prevents this. The obvious question is: "what is that historical issue" and, more importantly, how does it affect us today ? At the C.2.5 Parameters and Variables heading further down on that page there are a lot of examples of quoted expansions in what seems an exhaustive effort to list all of them, however, nowhere a twice double quoted example appear. ## Description The issue is related to the outputs of this 6 alternatives of quoting: - $b, "$b", ${a-$b}, ${a-"$b"} "${a-$b}" and "${a-"$b"}" The first five are defined by POSIX and some shells agree on the result of such expansions. Note that some values get split on the value of $IFS:

$ export a=abc; for sh in dash ksh93 bash; do $sh -c 'IFS="bl"; b=value 
  printf " " 1$b 2"$b" 3${a-$b} 4${a-"$b"} 5"${a-$b}" 6"${a-"$b"}";
  echo'; done

However, is "${1-"$var"}" defined somewhere in the spec ? That is: with **repeated** double quotes, external and internal. ## Additional 1 Just as related information, look at this results: Note that the only difference of the previous code to this next code is that $a is unset:

$ unset a; for sh in dash ksh93 bash; do $sh -c 'IFS="bl"; b=value 
  printf " " 1$b 2"$b" 3${a-$b} 4${a-"$b"} 5"${a-$b}" 6"${a-"$b"}";
  echo'; done

## Additional 2 Common shells act in conflicting ways when actually testing all quoting options.

$ for sh in dash ksh93 bash; do $sh -c 'IFS="bl"; b=value 
  printf " " 1\n 2"\n" 3${a-\n} 4${a-"\n"} 5"${a-\n}" 6"${a-"\n"}";
  echo'; done

And that is without including zsh!.