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Make the spec specific about the return type of sync*/async/async* function.

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Now requires the type to be a supertype of `Iterable<T>`/`Future<T>`/`Stream<T>`
for some `T`. This allows, say, all `Future<T>` return types for `async` but
disallows using a different class implementing `Future`:
```
MyFuture foo() async {}
```
gives a static warning (error in Dart 2) because the return type is statically
known to be unsatisfied.

Fixes issue #27470
BUG= http://dartbugcom/27470
R=eernst@google.com, floitsch@google.com

Review URL: https://codereview.chromium.org/2392513002 .
This commit is contained in:
Lasse R.H. Nielsen 2016-10-05 10:24:10 +02:00
parent eabd4ed9fb
commit 16d806b0aa
1 changed files with 3 additions and 1 deletions
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4
docs/language/dartLangSpec.tex
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@ -592,7 +592,9 @@ An asynchronous constructor would, by definition, never return an instance of th
One could allow modifiers for factories. A factory for \code{Future} could be modified by \ASYNC{}, a factory for \code{Stream} could be modified by \ASYNC* and a factory for \code{Iterable} could be modified by \SYNC*. No other scenario makes sense because the object returned by the factory would be of the wrong type. This situation is very unusual so it is not worth making an exception to the general rule for constructors in order to allow it.
}
\LMHash{}
It is a static warning if the declared return type of a function marked \ASYNC{} may not be assigned to \code{Future}. It is a static warning if the declared return type of a function marked \SYNC* may not be assigned to \code{Iterable}. It is a static warning if the declared return type of a function marked \ASYNC* may not be assigned to \code{Stream}.
It is a static warning if the declared return type of a function marked \ASYNC{} is not a supertype of \code{Future$<$\mbox{$T$}$>$} for some type $T$.
It is a static warning if the declared return type of a function marked \SYNC* is not a supertype of \code{Iterable$<$\mbox{$T$}$>$} for some type $T$.
It is a static warning if the declared return type of a function marked \ASYNC* is not a supertype of \code{Stream$<$\mbox{$T$}$>$} for some type $T$.
\subsection{Function Declarations}
\LMLabel{functionDeclarations}