Fix ICE (issue 8262).

FIxes: https://issues.dlang.org/show_bug.cgi?id=8262

This PR is still incomplete, as it still doesn't quite implement the `alias this` in the expected manner (the output of the test program in the bug is `S(0)` as opposed to `0`, which would be the case if the extra level of indirection via `Seq!s` was removed), and the error appears to be swallowed. At least the compiler won't segfault anymore, but probably this isn't the correct fix for the problem.