fix #12451 by providing a better error message

[rened]

No description provided.

[mbauman]

Instead of fixing up the thrown error with a try/catch block (which is fairly expensive), it'd be better to detect the error state itself before it occurs.

I'm not sure that you can do much about non-iterables (that's probably solved by a more informative error display at the REPL for missing start methods), but you could ensure that each element is itself an iterable collection of 2 elements instead of using automatic destructing in the for loop.

[rened]

@mbauman try/catch: true - my rationale was that running that check every time a Dict is constructed is also expensive (e.g. for many elements), so rather only check on error.

So better check for start/next/done first, then construct and let it fail hard for non-iterators? I'll check a few versions for speed.

[mbauman]

A few performance spot-checks would be ideal. At the very least, you can move this into the catchall dict_with_eltype (or inner constructor) for collections that aren't a bunch of Tuples or Pairs, since those won't have this problem.