[Bug c++/66659] New: Accepts invalid when undeduced context encountered deducing from a trailing parameter pack

["hstong at ca dot ibm.com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66659

            Bug ID: 66659
           Summary: Accepts invalid when undeduced context encountered
                    deducing from a trailing parameter pack
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hstong at ca dot ibm.com
  Target Milestone: ---

In the call to "f" from "h" below, the deduction for T@0 should find both
"fptype" and "Func" and fail.

### SOURCE (<stdin>):
template <typename ...T> struct A;

void g();
void g(bool);

typedef void (*fptype)();
struct Func { operator fptype(); } func;

template<typename ...T> void f(A<T ...> *ap, T ...b);

void h(A<fptype, fptype> *ap) { f(ap, func, g); }


### COMPILER INVOCATION:
g++ -std=c++11 -c -x c++ -


### ACTUAL OUTPUT:
(Clean compile).


### EXPECTED OUTPUT:
(Error message).


### COMPILER VERSION INFO:
Using built-in specs.
COLLECT_GCC=/usr/local/gcc-head/bin/g++
COLLECT_LTO_WRAPPER=/usr/local/gcc-head/libexec/gcc/x86_64-unknown-linux-gnu/6.0.0/lto-wrapper
Target: x86_64-unknown-linux-gnu
Configured with: /home/heads/gcc/gcc-source/configure
--prefix=/usr/local/gcc-head --enable-languages=c,c++ --enable-lto
--disable-multilib --without-ppl --without-cloog-ppl --enable-checking=release
--disable-nls
Thread model: posix
gcc version 6.0.0 20150623 (experimental) (GCC)