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

[Teodor Dutu]

On Friday, 2 September 2022 at 18:02:22 UTC, max haughton wrote:
> Try running gdb in follow fork mode, or run dub in verbose mode 
> to get the dmd command.
>
> The backends knowledge of where things come from is terrible.
>
> For debugging the backend if it's not an obvious segfault or 
> memory corruption etc you better get lucky with a printf or 
> you're out of luck. It's an old and horrible codebase, the 
> trees are simple enough but the algorithms were mostly designed 
> before anything resembling modern compiler design practice.
>
> Feel free to ramble on the d slack about what you've tried so 
> far.

Many thanks for the tips! I managed to debug the code and figured 
out that the issue was not necessarily caused by my PR, but 
merely exposed by it. I reduced the source of the bug to roughly 
the code below:
```d
Tarr foo(Tarr : T[], T)(return scope Tarr x) { return x; }

struct S
{
     import std.container.array : Array;
     Array!int[] x;

     Array!int[] bar()
     {
   	return (foo(this.x[]), this.x[]);
     }
}
```
[My 
lowering](https://github.com/dlang/dmd/blob/7cef3b9f3c43151c5def069a5f281088f561d370/compiler/src/dmd/expressionsem.d#L10054) created a `CommaExp` similar to `foo(this.x[]), this.x[]`. The second `this.x[]` caused the backend to think that the symbol `this` was defined multiple times.

However, I cannot create a bug report with the above code, 
because it does not compile in this form. `foo(this.x[]), 
this.x[]` issues: `Error: Using the result of a comma expression 
is not allowed`.

How would you recommend that I report this bug so it's easier to 
reproduce?