trap RETURN not set in calling function is overwritten by callee

[jtmoon+bashbug]

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../. -I.././include -I.././lib  -D_FORTIFY_SOURCE=2 -g 
-O2 -fstack-protector --param=ssp-buffer-size=4 -Wformat 
-Werror=format-security -Wall
uname output: Linux moonrock 3.13.0-48-generic #80-Ubuntu SMP Thu Mar 12 
11:16:15 UTC 2015 x86_64 x86_64 x86_64 GNU/Linux
Machine Type: x86_64-pc-linux-gnu

Bash Version: 4.3
Patch Level: 11
Release Status: release

Description:
        Given two functions wherein only one function sets a RETURN trap.  The 
other
        function will mistakenly call the other's RETURN trap upon it's return.

Repeat-By:
        # test trap RETURN if unset from called function
        function testU1() {
            function _testU1_ret() { echo "${FUNCNAME}" ; }
            trap _testU1_ret RETURN
        }
        function testU2() {
            testU1
            # when testU2 returns, will it call _testU1_ret ?
        }
        testU2
        # prints the following
        _testU1_ret
        _testU1_ret

Fix:
        For some reason the RETURN traps are not being stacked.  Instead, the
        current RETURN trap is just overwritten.