Fix #2762: Make sure constrainResult does not change context when false #2892
Conversation
constrainResult should behave like isSubType in this respect. This fixes scala#2672. The assertion in Applications.scala is no longer needed because it is now obviously true by design.
| case _ => | ||
| true | ||
| def constrainResult(mt: Type, pt: Type)(implicit ctx: Context): Boolean = { | ||
| val savedConstraint = ctx.typerState.constraint |
There was a problem hiding this comment.
Wouldn't it be safer to do use a new TyperState? (ctx.fresh.setNewTyperState and then commit if we want to keep it)
There was a problem hiding this comment.
Yes, except that constrainResult is called often, so copying the context each time would likely be a hit in performance. Also, the logic here is analogous to isSubType, which makes sense.
| @@ -221,7 +221,7 @@ trait Applications extends Compatibility { self: Typer with Dynamic => | |||
| if (ctx.typerState.isCommittable) | |||
| // defer the problem until after the application; | |||
| // it might be healed by an implicit conversion | |||
| assert(ctx.typerState.constraint eq savedConstraint) | |||
There was a problem hiding this comment.
If this is removed then val savedConstraint above is unused.
| def foo()(implicit ev: Prepend): Foo[ev.Out] = ??? | ||
|
|
||
| def test: Unit = { | ||
| foo(): Foo[Any] // error: found: Prepend => Foo[_] required: Foo[Any] |
There was a problem hiding this comment.
This isn't a great error message by the way, since it might be displayed instead of a more informative message based on@implicitNotFound.
There was a problem hiding this comment.
But even if an implicit was found, we'd still get an error because then the function would return a Foo[_] where a Foo[Any] was expected.
constrainResult should behave like isSubType in this respect.
This fixes #2672. The assertion in Applications.scala is no
longer needed because it is now obviously true by design.