Help with failing assert in dmd.backend.symbol.d

[Teodor Dutu]

Hi,

I have recently opened [this 
PR](https://github.com/dlang/dmd/pull/14382) to change 
`_d_arraysetassign` to a template. It has caused [this builkite 
failure in 
vibe-d](https://buildkite.com/dlang/dmd/builds/27898#0182f34a-7f4a-49f9-8b93-01e5ba42998b). I am able to reproduced it locally, but I am unable to debug it.

Being a backend error, the `Symbol` struct doesn't seem to store 
precise information about the line and file from where that 
symbol comes. I have used `Symbol.lposscopestart`, but that only 
tells where the symbol was created. In my case, it was created 
here: `phobos/std/typecons.d(6822)`, but this doesn't provide me 
with too much info.

How would you debug this? Do you have any "tips" for debugging 
the backend? I'd also like to build vibe-d without dub so I can 
trace the compiler directly. How can I do this?

Thanks for the help,
Teodor