How to interpret the template function signature of backtrace in GDB?

The output of backtrace of GDB is pretty messy, especially for template.

For instance:

Thread 2 (LWP 100146 of process 1245):
#0  thr_new () at thr_new.S:3
#1  0x000000080025c3da in _pthread_create (thread=0x7fffdfffd880, attr=<optimized out>, start_routine=0x205500 <void* std::__1::__thread_proxy<std::__1::tuple<std::__1::unique_ptr<std::__1::__thread_struct, std::__1::default_delete<std::__1::__thread_struct> >, void (std::__1::__async_assoc_state<void, std::__1::__async_func<func2(std::__1::atomic<long>&)::$_0> >::*)(), std::__1::__async_assoc_state<void, std::__1::__async_func<func2(std::__1::atomic<long>&)::$_0> >*> >(void*)>, arg=0x8007fa8e0) at /usr/src/lib/libthr/thread/thr_create.c:188
#2  0x0000000000204e40 in std::__1::thread::thread<void (std::__1::__async_assoc_state<void, std::__1::__async_func<func2(std::__1::atomic<long>&)::$_0> >::*)(), std::__1::__async_assoc_state<void, std::__1::__async_func<func2(std::__1::atomic<long>&)::$_0> >*, void>(void (std::__1::__async_assoc_state<void, std::__1::__async_func<func2(std::__1::atomic<long>&)::$_0> >::*&&)(), std::__1::__async_assoc_state<void, std::__1::__async_func<func2(std::__1::atomic<long>&)::$_0> >*&&) ()
#3  0x0000000000204309 in std::__1::future<void> std::__1::__make_async_assoc_state<void, std::__1::__async_func<func2(std::__1::atomic<long>&)::$_0> >(std::__1::__async_func<func2(std::__1::atomic<long>&)::$_0>&&) ()
#4  0x00000000002035ea in std::__1::future<std::__1::__invoke_of<std::__1::decay<func2(std::__1::atomic<long>&)::$_0>::type>::type> std::__1::async<func2(std::__1::atomic<long>&)::$_0>(std::__1::launch, func2(std::__1::atomic<long>&)::$_0&&) ()
#5  0x0000000000203462 in func2(std::__1::atomic<long>&) ()
#6  0x0000000000206f18 in main::$_1::operator()() const ()
#7  0x0000000000206eed in void std::__1::__async_func<main::$_1>::__execute<>(std::__1::__tuple_indices<>) ()
#8  0x0000000000206ea5 in std::__1::__async_func<main::$_1>::operator()() ()
#9  0x0000000000206df3 in std::__1::__async_assoc_state<void, std::__1::__async_func<main::$_1> >::__execute() ()
#10 0x0000000000207183 in void* std::__1::__thread_proxy<std::__1::tuple<std::__1::unique_ptr<std::__1::__thread_struct, std::__1::default_delete<std::__1::__thread_struct> >, void (std::__1::__async_assoc_state<void, std::__1::__async_func<main::$_1> >::*)(), std::__1::__async_assoc_state<void, std::__1::__async_func<main::$_1> >*> >(void*) ()
#11 0x000000080025c776 in thread_start (curthread=0x8007de500) at /usr/src/lib/libthr/thread/thr_create.c:292
#12 0x0000000000000000 in ?? ()
Backtrace stopped: Cannot access memory at address 0x7fffdfffe000

In frame #8, there are three pairs of parentheses, std::__1::__async_func<main::$_1>::operator()() () what do they mean exactly?