Deduction guides and variadic class templates with variadic template constructors - mismatched argument pack lengths

Question

Consider the following class definition and deduction guide:

template <typename... Ts>
struct foo : Ts...
{
    template <typename... Us>
    foo(Us&&... us) : Ts{us}... { }
};

template <typename... Us>
foo(Us&&... us) -> foo<Us...>;

---

If I try to instantiate foo with explicit template arguments, the code compiles correctly:

foo<bar> a{bar{}}; // ok

---

If I try to instantiate foo through the deduction guide...

foo b{bar{}};

• g++7 produces a compiler error:

  prog.cc: In instantiation of 'foo<Ts>::foo(Us ...) [with Us = {bar}; Ts = {}]':
  prog.cc:15:16:   required from here
  prog.cc:5:27: error: mismatched argument pack lengths while expanding 'Ts'
       foo(Us... us) : Ts{us}... { }
                             ^~~
  

• clang++5 explodes:

  #0 0x0000000001944af4 PrintStackTraceSignalHandler(void*) (/opt/wandbox/clang-head/bin/clang-5.0+0x1944af4)
  #1 0x0000000001944dc6 SignalHandler(int) (/opt/wandbox/clang-head/bin/clang-5.0+0x1944dc6)
  #2 0x00007fafb639a390 __restore_rt (/lib/x86_64-linux-gnu/libpthread.so.0+0x11390)
  #3 0x0000000003015b30 clang::Decl::setDeclContext(clang::DeclContext*) (/opt/wandbox/clang-head/bin/clang-5.0+0x3015b30)
  ...
  clang-5.0: error: unable to execute command: Segmentation fault
  

live example on wandbox

While clang++ is definitely bugged (reported as issue #32673), is g++ correct in rejecting my code? Is my code ill-formed?

Answer 1

To simplify your example further, it appears that GCC does not implement variadic template arguments in deduction guides:

https://wandbox.org/permlink/4YsacnW9wYcoceDH

I didn't see any explicit mention of variadic templates in the wording for deduction guides in the standard or on cppreference.com. I see no interpretation of the standard that disallows this. Therefore I think this is a bug.