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TypeTag disables implicit conversion #9680

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scabug opened this issue Mar 2, 2016 · 3 comments
Open

TypeTag disables implicit conversion #9680

scabug opened this issue Mar 2, 2016 · 3 comments
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implicit infer typer
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@scabug
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scabug commented Mar 2, 2016

This "bug" came up in a Stackoverflow question.
When you put a subtype constraint on a type parameter in an implicit conversion, the conversion is also applied when an unrelated type is involved. My guess is that Nothing is involved here somewhere. But when I want to check the type with a TypeTag, the conversion no longer kicks in for the unrelated type, and for the right type I get "error: macro has not been expanded".

scala> trait TestBase
defined trait TestBase

scala> class NotTest
defined class NotTest

scala> case class Foo()
defined class Foo

scala> implicit def conv[A <: TestBase](foo: Foo): A = {println(foo); null.asInstanceOf[A] }
warning: there were 1 feature warning(s); re-run with -feature for details
conv: [A <: TestBase](foo: Foo)A

scala> val test: NotTest = Foo()
Foo()
test: NotTest = null

scala> implicit def conv[A](foo: Foo)(implicit ev: A <:< TestBase): A = {println(foo); null.asInstanceOf[A] }
warning: there were 1 feature warning(s); re-run with -feature for details
conv: [A](foo: Foo)(implicit ev: <:<[A,TestBase])A

scala> val test: NotTest = Foo()
Foo()
test: NotTest = null

scala> import scala.reflect.runtime.universe._
import scala.reflect.runtime.universe._

scala> implicit def conv[A <: TestBase](foo: Foo)(implicit ev: TypeTag[A]): A = {println(ev); null.asInstanceOf[A] }
warning: there were 1 feature warning(s); re-run with -feature for details
conv: [A <: TestBase](foo: Foo)(implicit ev: reflect.runtime.universe.TypeTag[A])A

scala> val test: NotTest = Foo()
<console>:18: error: type mismatch;
 found   : Foo
 required: NotTest
       val test: NotTest = Foo()
                              ^

scala> val test: TestBase = Foo()
<console>:17: error: macro has not been expanded
       val test: TestBase = Foo()
                               ^
@scabug
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scabug commented Mar 2, 2016

Imported From: https://issues.scala-lang.org/browse/SI-9680?orig=1
Reporter: Jasper-M
Affected Versions: 2.10.4, 2.11.7
See #9633, #7608

@scabug
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scabug commented Mar 2, 2016

@retronym said:
I'd suggest using -Xprint:typer to see what is inferred, rather than adding TypeTag-s. This might help you to untangle the issues better and determine if this is a bug or just a case of inference doing what it specced to do.

@scabug
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scabug commented Mar 3, 2016

@szeiger said:
The first part works as expected (as -Xprint:typer will verify). The implicit conversion claims to be able to produce any subtype of TestBase, which includes Nothing. Since you need a cast to get there, it's not a soundness issue, either (but the similar #9633 is). You'd have to add a subtype of all the types the implicit conversion can really produce, and use that as an upper bound.

The fact that it doesn't work with a TypeTag looks like a legitimate bug to me. An explicit call conv(Foo()) compiles but conv is not picked as an implicit conversion.

@scabug scabug added this to the Backlog milestone Apr 7, 2017
@Jasper-M Jasper-M mentioned this issue Jan 15, 2018
Open
@Jasper-M Jasper-M changed the title Implicit conversion disregards subtype constraints. TypeTag disables implicit conversion Jan 15, 2018
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