Compilation error on OneLiner

[Segebre]

When using fold inside a print() we get a compilation error for the + inside the combine function. This happens on DartPad and can be shown here.

[lrhn]

The code in question is

var nums = [1, 4, 2, 3, 5];
print(nums.fold(-10, (p, i) => p + i)); // Uncommenting this causes a compilation error.

And the error is that + is not defined on Object.
I'm guessing downards inference pushes down Object as the return type of fold, which then means that the type of p becomes Object.
The signature of fold is:

class Iterable<E> {
   T fold<T>(T intiialValue, T Function(T, E) combine));
}

This is technically working as intended, because the downwards inference wins. It's just a useless inference here, and a place where upwards would give the correct result.

[eernstg]

Also note #25490 and #34118, where it is discussed that fold in particular tends to create situations where the current approach to inference does not route the available information far enough to produce the best outcome. Changes in this area need to be made cautiously because they are likely to be pervasively but subtly breaking, but it is certainly on the radar.

I believe the consistent approach would be to close this issue as a duplicate of #25490. I've added a reference to this issue in #25490 in order to preserve the information that it came up again, and how.