Unable to generate bindings: ()

I am using this script https://gist.github.com/fitzgen/187381e358f60efa8194d0b276b4d11a.

The hashtag for my bindgen version is 4dd4ac7 .
The header file compiles with clang++ -std=c++11

$ ./b.sh bindgen abc.h

clang-4.0: warning: treating 'c-header' input as 'c++-header' when in C++ mode, this behavior is deprecated [-Wdeprecated]
abc.h:2:11: error: no member named 'c' in 'a<int>::(anonymous union at abc.h:3:3)'
abc.h:6:18: note: in instantiation of member function 'a<int>::a' requested here
abc.h:2:11: error: no member named 'c' in 'a<int>::(anonymous union at abc.h:3:3)', err: true
thread 'main' panicked at 'Unable to generate bindings: ()', /checkout/src/libcore/result.rs:860:4
stack backtrace:
   0: std::sys::imp::backtrace::tracing::imp::unwind_backtrace
             at /checkout/src/libstd/sys/unix/backtrace/tracing/gcc_s.rs:49
   1: std::sys_common::backtrace::_print
             at /checkout/src/libstd/sys_common/backtrace.rs:71
   2: std::panicking::default_hook::{{closure}}
             at /checkout/src/libstd/sys_common/backtrace.rs:60
             at /checkout/src/libstd/panicking.rs:380
   3: std::panicking::default_hook
             at /checkout/src/libstd/panicking.rs:396
   4: std::panicking::rust_panic_with_hook
             at /checkout/src/libstd/panicking.rs:611
   5: std::panicking::begin_panic_new
             at /checkout/src/libstd/panicking.rs:553
   6: std::panicking::begin_panic_fmt
             at /checkout/src/libstd/panicking.rs:521
   7: rust_begin_unwind
             at /checkout/src/libstd/panicking.rs:497
   8: core::panicking::panic_fmt
             at /checkout/src/libcore/panicking.rs:92
   9: core::result::unwrap_failed
             at /checkout/src/libcore/macros.rs:41
  10: <core::result::Result<T, E>>::expect
             at /checkout/src/libcore/result.rs:762
  11: bindgen::main::{{closure}}
             at src/main.rs:55
  12: std::panicking::try::do_call
             at /checkout/src/libstd/panicking.rs:479
  13: __rust_maybe_catch_panic
             at /checkout/src/libpanic_unwind/lib.rs:98
  14: std::panicking::try
             at /checkout/src/libstd/panicking.rs:458
  15: std::panic::catch_unwind
             at /checkout/src/libstd/panic.rs:361
  16: bindgen::main
             at src/main.rs:54
  17: __rust_maybe_catch_panic
             at /checkout/src/libpanic_unwind/lib.rs:98
  18: std::rt::lang_start
             at /checkout/src/libstd/panicking.rs:458
             at /checkout/src/libstd/panic.rs:361
             at /checkout/src/libstd/rt.rs:59
  19: main
  20: __libc_start_main
  21: _start
Interesting: bindgen failed to generate bindings for the test case!

$ cat abc.h

template <typename = int> struct a {
  a() { b.c; }
  union {
  } b;
};
a<> d() { return {}; }

Reproduce the failure with the linked script, clang++ command, and abc.h, then inspect src/main.rs where the binding-generation result is unwrapped. Trace why the clang diagnostic for the empty anonymous union becomes the "Unable to generate bindings" panic. Done means the case is handled without this panic and its behavior is covered by a regression test.