inappropriate brace expansion in command substitution

[Greg Wooledge]

Someone in IRC was doing an overly complex command invovling grep "^.{1,8}"
inside a command substitution, and we believe he stumbled upon a bash bug.
I've simplified it down to this:

$ echo "$(echo "a{1,8}")"
a1 a8

The curly braces here should not be treated as a brace expansion, since
they're obviously inside a quoted argument.

This is occurring in every version of bash tested (which includes 2.05b,
3.2.51, 4.0.33, 4.1.9, and 4.2.20).

This slightly different command works as expected:

x="$(echo "a{1,8}")"; echo "$x"
a{1,8}