Accepts-invalid with dependent name not prefixed with typename

[efriedma-quic]

Bugzilla Link	16909
Version	unspecified
OS	Windows NT
CC	@hyp,@benlangmuir,@DougGregor

Extended Description

Testcase:

template <typename,typename>
struct base {
  template <typename> struct derived;
};
template <typename T, typename U, typename V> base<T, U>::derived<V> foo();

We should print an error about the missing typename and template keywords.

[benlangmuir]

This can also cause assertion failures or generate invalid IR:

template<typename T> struct A {
  template<typename U> struct B {};
};
struct C {};
template<typename T> A<T>::B<T> begin(const T &);

void test() {
  auto __begin = begin(5); // asserts because we didn't deduce the type of __begin
  for (auto &&x : C()); // without assertions, will produce invalid LLVM IR
}

[llvmbot]

This is also tracked as rdar://problem/19438432.

Ben proposed a patch:
http://lists.llvm.org/pipermail/cfe-commits/Week-of-Mon-20150126/122055.html

[Endilll]

Diagnostics is emitted since Clang 5: https://godbolt.org/z/1K786Tan4
Crash doesn't reproduce with assertion builds of Clang 3.5 and 3.6: https://godbolt.org/z/dErrdM4hd

[efriedma-quic]

The following still crashes:

template<typename T> struct A {
  template<typename U> struct B {};
};
struct C {};
template<typename T> A<T>::B<T> begin(const T &);

void test() {
  auto __begin = begin(5); // asserts because we didn't deduce the type of __begin
}

[Endilll]

Crash appears to be fixed in Clang 5: https://godbolt.org/z/bbsvE68MT
But it's back since Clang 16: https://godbolt.org/z/hMMofvGM7