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
}

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

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

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
}

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

@llvm/issue-subscribers-clang-frontend

I think what happened is that we added a check to reject this... but when we implemented the C++20 typename rules in https://reviews.llvm.org/D53847, the check was relaxed, so this issue showed up again.

CC @alanzhao1 @cor3ntin @erichkeane