Ternary operator type checking

[blackmenthor]

Thank you for taking the time to file an issue!

In order to route, prioritize, and act on this, please include:

• Dart version 2.1.0-dev.5.0.flutter-a2eb050044
• MacOSX
• on Flutter version 0.9.4

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So i accidentally wrote this code

int point = snapshot.hasData ? snapshot.data < 0 ? "-" : snapshot.data : "-";

and somehow i able to insert a string value to the int variable and it produced no error whatsoever. But, when it ran on the users phone it produce error

_TypeError
type '_OneByteString' is not a subtype of type 'int'

I think somehow dart think that this expression will not produced an error so it compiles successfully.

But in fact it will produced an error because i inserted a string value to an int.

Or maybe the problem lies on Android studio that i used?

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[eernstg]

This is a known issue with (1) the typing of a conditional expression (it's a least-upper-bound-ish result of combining the types of the two branches) and (2) an implicit downcast.

Given that you're using snapshot.data < 0 to make a decision it seems likely that the type of snapshot.data is int (but it could also be dynamic). The static type of the initializing expression is then a top type, that is, a type which is a supertype of all other types (if snapshot.data has type dynamic then that's it, but if data has type int then we get Object). In any case, it is a supertype of int.

Then you're using that expression to initialize a variable of type int, and the compiler notes that there is a downcast and generates code for a dynamic check.

If you had used something like int x = "hello"; then you would have had an error, because that's not a downcast.

If you want to prevent implicit downcasts then you can use option --no-implicit-casts. There's lively debate about making that setting the default (#31410) for Dart.

This particular situation is a well-known worst case for implicit casts, because there's an upcast to a top type like dynamic or Object, and then a downcast. And that downcast is considered viable for all target types, because they are all subtypes thereof. See #25368 and #34058 for earlier issues where this is discussed.

I'll close this one as a duplicate of #25368, such that this discussion can be kept in one place.