The source code in next report is completely valid source code but GCC
5.0.0 rejects to compile it.
Older version of GCC can compile this.
I tested this in Ubuntu but I've got the same report in fedora from other user.

If I change the type of varaible 'a' to int from const int, or make the test function non-template, it can be compiled.
-------------------------------------------------------------------------------

the exact version of GCC
5.0.0 20150329 (experimental) [trunk revision 221764] (Ubuntu
5-20150329-1ubuntu11~14.04)

the system type
x86_64-linux-gnu

the options given when GCC was configured/built
Configured with: ../src/configure -v --with-pkgversion='Ubuntu
5-20150329-1ubuntu11~14.04'
--with-bugurl=file:///usr/share/doc/gcc-5/README.Bugs
--enable-languages=c,ada,c++,java,go,fortran,objc,obj-c++
--prefix=/usr --program-suffix=-5 --enable-shared
--enable-linker-build-id --libexecdir=/usr/lib
--without-included-gettext --enable-threads=posix
--with-gxx-include-dir=/usr/include/c++/5 --libdir=/usr/lib
--enable-nls --with-sysroot=/ --enable-clocale=gnu
--enable-libstdcxx-debug --enable-libstdcxx-time=yes
--with-default-libstdcxx-abi=c++98 --enable-gnu-unique-object
--disable-vtable-verify --enable-libmpx --enable-plugin
--with-system-zlib --disable-browser-plugin --enable-java-awt=gtk
--enable-gtk-cairo
--with-java-home=/usr/lib/jvm/java-1.5.0-gcj-5-amd64/jre
--enable-java-home
--with-jvm-root-dir=/usr/lib/jvm/java-1.5.0-gcj-5-amd64
--with-jvm-jar-dir=/usr/lib/jvm-exports/java-1.5.0-gcj-5-amd64
--with-arch-directory=amd64
--with-ecj-jar=/usr/share/java/eclipse-ecj.jar --enable-objc-gc
--enable-multiarch --disable-werror --with-arch-32=i686 --with-abi=m64
--with-multilib-list=m32,m64,mx32 --enable-multilib
--with-tune=generic --enable-checking=yes --build=x86_64-linux-gnu
--host=x86_64-linux-gnu --target=x86_64-linux-gnu

the complete command line that triggers the bug
g++-5 -std=c++11 test.cpp

the compiler output (error messages, warnings, etc.)
test.cpp: In instantiation of 'test(T)::<lambda()> [with T = int]':
test.cpp:5:7:   required from 'struct test(T) [with T = int]::<lambda()>'
test.cpp:5:28:   required from 'void test(T) [with T = int]'
test.cpp:9:11:   required from here
test.cpp:5:27: error: redeclaration of 'const int& a'
     [&] () { return a, a; }();
                           ^
test.cpp:5:27: note: 'const int& a' previously declared here
test.cpp: In instantiation of 'void test(T) [with T = int]':
test.cpp:9:11:   required from here
test.cpp:5:5: sorry, unimplemented: non-trivial designated
initializers not supported
     [&] () { return a, a; }();

the preprocessed file (*.i*) that triggers the bug
# 1 "test.cpp"
# 1 "<built-in>"
# 1 "<command-line>"
# 1 "/usr/include/stdc-predef.h" 1 3 4
# 1 "<command-line>" 2
# 1 "test.cpp"
template<class T>
void test(T b)
{
    const int a = b;
    [&] () { return a, a; }();
}

int main() {
    test(1);
 return 0;
}