inconsistent $? after &

[Martin Kealey]

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-pc-linux-gnu'
-DCONF_VENDOR='pc' -DLOCALEDIR='/usr/share/locale' -DPACKAGE='bash' -DSHELL
-DHAVE_CONFIG_H   -I.  -I../bash -I../bash/include -I../bash/lib
-D_FORTIFY_SOURCE=2 -g -O2 -fstack-protector --param=ssp-buffer-size=4
-Wformat -Wformat-security -Werror=format-security -Wall
uname output: Linux treehug.home.kurahaupo.gen.nz 3.2.0-54-generic
#82-Ubuntu SMP Tue Sep 10 20:08:42 UTC 2013 x86_64 x86_64 x86_64 GNU/Linux
Machine Type: x86_64-pc-linux-gnu

Bash Version: 4.2
Patch Level: 25
Release Status: release

Description:
        The value of $? after starting a backgrounded command is
inconsistent:
        $? is unchanged after starting a sufficiently complex command, but
        after starting a simpler command it is set to 0.

        Determining "sufficiently complex" has been a bit tricky, but it
        appears to depend only on the type of the top-level node of the
        parse-tree:

        An unbracketed conditional chain of pipelines will leave $? set to 0
        regardless of the complexity of its components. Likewise invoking a
        function will leave $? set to 0 no matter how complex the body of
        the function may be.

        Conversely, grouping even a single simple command with brackets or
        using any control structure will leave $? untouched.

Repeat-By:
        ( (exit 42) ; :                    & echo $? ; wait )   # says 0
        ( (exit 42) ; : | : && : || :      & echo $? ; wait )   # says 0
        ( (exit 42) ; : | if :; then :; fi & echo $? ; wait )   # says 0
        ( (exit 42) ;     if :; then :; fi & echo $? ; wait )   # says 42
        ( (exit 42) ; { : ; }              & echo $? ; wait )   # says 42