Strange inference with a ternary operator and Lists

[nex3]

On the following code:

void fn(Iterable iter) {}

void main() {
  bool condition = true;
  Iterable<String> iter = [];
  fn(condition ? [] : iter.toList());
}

The analyzer reports the following warning:

[warning] The argument type 'EfficientLength' cannot be assigned to the parameter type 'Iterable'

This seems wrong. The first branch of the ternary operator is a List, which is assignable to Iterable.

Note that if <String> is removed from the type annotation for iter, or if <String> is added to the list literal, the warning goes away.

I'm using 1.13.0-edge.9e18a7519d429f87c70bb3b8871c6cc666ee3fa6.

[lrhn]

This looks like an instance of issue #19425.

[bwilkerson]

Yes, sadly, unless we get a different definition of least upper bounds we're stuck. The current specification of LUB says that LUB(List, List) is EfficientLength. The answer that everyone expects is List, so it surprises lots of people fairly frequently.

To work around this, you either have to write "[]" for the literal, or stop using the conditional operator (the only place where LUB is used in the spec.).