Quality of errors in DMD

[Steven Schveighoffer via Digitalmars-d]

On 9/2/16 10:26 AM, Ethan Watson wrote:
> Can we have a serious discussion in here about the quality of DMD errors?
>
> I've been alternately a dog chasing its own tail, and a dog barking at a
> fire hydrant, chasing down errors deep in templated and mixin code over
> the last day. This has resulted in manually reducing templates and
> mixins by hand until I get to the root of the problem, which then
> results in submitting a bug and devising an ugly workaround.
>
> And then I get this one in some code:
>
> Assertion failure: '0' on line 1492 in file 'glue.c'

This is an internal compiler error. It's not a standard way of reporting 
errors in D code. It means the internal state of the compiler is messed. 
Not much the compiler can do except crash.

>
> The problem ended up being that a symbol tucked away in a template that
> itself was tucked away in a template was undefined, but it couldn't tell
> me that. Rather, it just assert(0)'d and terminated. Rather less
> helpfully, the only meaningful information it could give me at the
> assert point (Could it add to it further down the stack? Maybe?) was
> defined out because DMD wasn't in a debug build.

What you need to do is submit a bug report with minimal example and tag 
as ice. It should have a high priority.

>
> Honestly, I find stuff like this in a compiler unacceptable. Using
> assert(0) as shorthand for an unexpected error is all fine and dandy
> until you put your product in the hands of the masses and they expect
> your program to at least give you some idea of what was going wrong
> rather than just crashing out in flames.
>
> So just for fun, I searched DMD for all instances of assert(0) in the
> code base.
>
> 830 matches in DMD 2.070.2.
>
> That's 830 possible places where the compiler will give the user
> virtually no help to track down what (if anything) they did wrong.

But these are sanity checks on the compiler internal state. All the 
compiler can do is print that something is wrong and where it detected 
that. It can't diagnose the error because that assert is not supposed to 
happen!

-Steve