extglob syntax error in function definition

[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-strong -Wformat -Werror=format-security -Wall
uname output: Linux ubak 3.13.0-30-generic #55-Ubuntu SMP Fri Jul 4 21:40:53 
UTC 2014 x86_64 x86_64 x86_64 GNU/Linux
Machine Type: x86_64-pc-linux-gnu

Bash Version: 4.3
Patch Level: 30
Release Status: release

Description:
        The shell gives a syntax error when defining a function that uses the 
extended pattern matching operators.

        The man page does not mentin that extglob needs to be set at all. It 
does only say that it is needed for the !(...) construct.

        But i reckon then that setting extglob is also of influence on the 
other operators: ?(..) , *(...) , +(...) and @(...) .
        And indeed it is.

        This works:
                ## assuming extglob is off by default
                shopt -s extglob
                isnum () { case "$1" in  [1-9]*([0-9])) return 0 ;; *) return 1 
;; esac; }
                shopt -u extglob
        
        Drawback is that the global extglob setting is being manipulated in the 
current execution environment.

        I would have expected that i could encapsulate the setting of extglob, 
by using a subshell-like function:
                shopt -u extglob
                isnum () ( shopt -s extglob; case "$1" in  [1-9]*([0-9])) 
return 0 ;; *) return 1 ;; esac; )
        
        But unfortunately this gives a syntax error.
        It seems that the parser does not, at definition time, recognize the 
extended pattern matching construct.

Repeat-By:
        shopt -u extglob
        isnum () ( shopt -s extglob; case "$1" in  [1-9]*([0-9])) return 0 ;; 
*) return 1 ;; esac; )