It's not valid code (maybe it can be made valid, but I'm not sure how).
To trigger the crash fixed in r243987 requires a friend function with
more than four template parameter lists. With this test we have at least
some coverage.
git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@243989 91177308-0d34-0410-b5e6-96231b3b80d8
Benjamin Kramer