"$(echo "x'" '{1,2}')" performs brace expansion, even though it should not

[Marco Elver]

Configuration Information [Automatically generated, do not change]:
Machine: x86_64
OS: linux-gnu
Compiler: gcc
Compilation CFLAGS:  -DPROGRAM='bash' -DCONF_HOSTTYPE='x86_64' 
-DCONF_OSTYPE='linux-gnu' -DCONF_MACHTYPE='x86_64-unknown-linux-gnu' 
-DCONF_VENDOR='unknown' -DLOCALEDIR='/usr/share/locale' -DPACKAGE='bash' 
-DSHELL -DHAVE_CONFIG_H   -I.  -I. -I./include -I./lib   -march=x86-64 
-mtune=generic -O2 -pipe -fstack-protector --param=ssp-buffer-size=4 
-D_FORTIFY_SOURCE=2 
-DDEFAULT_PATH_VALUE='/usr/local/bin:/usr/bin:/bin:/usr/local/sbin:/usr/sbin:/sbin'
 -DSTANDARD_UTILS_PATH='/usr/bin:/bin:/usr/sbin:/sbin' 
-DSYS_BASHRC='/etc/bash.bashrc' -DSYS_BASH_LOGOUT='/etc/bash.bash_logout'
uname output: Linux laptop 3.6.11-1-ARCH #1 SMP PREEMPT Tue Dec 18 08:57:15 CET 
2012 x86_64 GNU/Linux
Machine Type: x86_64-unknown-linux-gnu

Bash Version: 4.2
Patch Level: 42
Release Status: release

Description:
        If a double-quoted command substitution command contains a
        double-quoted string containing a single-quote, followed by a
        single-quoted string containing a brace-expansion expression,
        then the command to be substituted is not executed correctly.

        See examples below.

        Also tested with the following versions and produces the same behaviour:
        GNU bash, version 3.2.25(1)-release (x86_64-redhat-linux-gnu)

Repeat-By:
        $ echo "$(echo "x" '{1,2}')" # Produces expected result
        x {1,2}
        $ echo "$(echo "x'" '{1,2}')" # Bug case: Add single-quote to first 
string, causing brace-expansion!
        x' 1 x' 2
        $ echo $(echo "x'" '{1,2}') # Produces expected result, but unquoted!
        x' {1,2}