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I like this idea. Kasper had a similar idea of checking that a type is Iterable for for-each statements.

Marking Later/ Enhancement/ Hint.

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Unfortunately, the specification says that the for-in statement is equivalent to

var n0 = e.iterator;
while (n0.moveNext()) {
   finalConstVarOrType? id = n0.current;
   s
}

The use of 'var' means that the static type of 'n0.current' is dynamic, which is assignable to 'int'. I have opened issue #18552 to see whether it's possible to change this.

FWIW, even if the specification is not changed, I think it would still be ok for us to give a hint in this situation. (Just because one construct is syntactic sugar for another, doesn't mean that we have to restrict ourselves to generating hints based on the de-sugared result).

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This is done now, as of: cb7aa02