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Declaration order breaks implicit resolution in subclass #10220

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scabug opened this issue Mar 7, 2017 · 2 comments
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Declaration order breaks implicit resolution in subclass #10220

scabug opened this issue Mar 7, 2017 · 2 comments
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@scabug
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scabug commented Mar 7, 2017

I have seen a number of implicit resolution issues that have to do with declaration order. But they all seem to occur in fairly esoteric examples. I have what I think is a very normal example:

class B extends A { implicitly[Int] }
class A { implicit val i = 3 }

error: could not find implicit value for parameter e: Int
       class B extends A { implicitly[Int] }

It works in the reverse order:

class A { implicit val i = 3 }
class B extends A { implicitly[Int] }

A funny thing is, if you try to work around it by bringing the implicit into scope manually, you get an error because you have two implicits:

class B extends A { val x = { implicit val j = i; implicitly[Int] } }
class A { implicit val i = 3 }

error: ambiguous implicit values:
 both value i in class A of type => Int
 and value j of type Int
 match expected type Int
       class B extends A { val x = { implicit val j = i; implicitly[Int] } }
                                                                   ^

But if you delete the implicit val j = i line then it cannot find the implicit.
@scabug
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scabug commented Mar 7, 2017

Imported From: https://issues.scala-lang.org/browse/SI-10220?orig=1
Reporter: Daniel Darabos (darabos)
Affected Versions: 2.11.6, 2.11.8, 2.12.1
Duplicates #8697

@scabug
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scabug commented Mar 7, 2017

@som-snytt said:
Isn't this just the requirement that implicits defined later in the source file must have an explicit type annotation.

class B extends A { implicitly[Int] }
class A { implicit val i: Int = 3 }

@scabug scabug closed this as completed Mar 7, 2017
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