named arguments can't be used for implicit extension method if original type has same-named method

[scabug]

class A {
  def a(s: String) {}
}

object O {
  implicit class B(val a: A) {
    def a(i: Int) {}
    def b(i: Int) {}
  }
}

import O._

new A().a("")    // works
new A().a(1)     // works
new A().b(i = 1) // works
new A().a(i = 1) // does not work

The last line gives the following error message (Scala 2.10.2):
<console>:25: error: not found: value i
              new A().a(i = 1) // does not work

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-7809?orig=1
Reporter: @retronym
Affected Versions: 2.10.0, 2.11.8, 2.12.1

[scabug]

Justin Bailey (m4dc4p) said (edited on Mar 25, 2017 8:43:54 PM UTC):
To motivate fixing this bug, consider the following - automatically converting nullable types to Option:

case class AuthorizationRequest(
  val sid: String,
  val oid: Option[String],
  val o_class: Option[String],
  val act: Option[String],
  val ctx: Option[String]
)

trait AuthorizationClient {
  def authorize(req: AuthorizationRequest): Task[Unit]
}

object AuthorizationClient {
  implicit class AuthorizationExts(client: AuthorizationClient) {
    def authorize(sid: String, oid: String = null, o_class: String = null, act: String = null, ctx: Json = null) = {
      client.authorize(AuthorizationRequest(sid = sid, oid = Option(oid), o_class = Option(o_class), act = Option(action), ctx = Option(ctx)))
    }
  }
}

The intermediate Request type just adds verbosity, so the authorize method on AuthorizationExts makes it easy to call authorize. But authorize is long enough (and its arguments have the same type) that it would be nice to call it with named arguments:

auth.authorize(sid ="foo", oid = "bar", o_class = "bif", etc.) 

This also makes it impossible to omit parameters and use defaults instead. Self-contained example in my next comment.

[scabug]

Justin Bailey (m4dc4p) said (edited on Mar 25, 2017 8:50:12 PM UTC):
Here is a minimal example:

object S {
  class Test {
    def test(arg: Int) = ???
  }

  class ExtClass(f: Test) {
    def test(arg: String) = ???
  }

  implicit final def Implicit(t: Test) = new ExtClass(t)

  (new Test).test("works")
  (new Test).test(arg = "fails")
}

Note that test("works") compiles, while test(arg = "fails") does not.

I ran with -Ytyper-debug (2.12.2). Full output below, but I believe the key line (from test(arg = "fails")) is this:

no second try: (arg: Int)Nothing and arg = "fails" because error not in result: source-C:\Users\Justin\Source\SI-7809\S.scala,line-13,offset=246!=source-C:\Users\Justin\Source\SI-7809\S.scala,line-13,offset=239

This is the full typer-debug output for test(arg = "fails"):

|    |-- new Test().test(arg = "fails") BYVALmode-EXPRmode (site: value <local S> in S)
|    |    |-- new Test().test BYVALmode-EXPRmode-FUNmode-POLYmode (silent: value <local S> in S)
|    |    |    |-- new Test() EXPRmode-POLYmode-QUALmode (silent: value <local S> in S)
|    |    |    |    |-- new Test BYVALmode-EXPRmode-FUNmode-POLYmode (silent: value <local S> in S)
|    |    |    |    |    |-- new Test EXPRmode-POLYmode-QUALmode (silent: value <local S> in S)
|    |    |    |    |    |    |-- Test FUNmode-TYPEmode (silent: value <local S> in S)
|    |    |    |    |    |    |    \-> S.Test
|    |    |    |    |    |    \-> S.Test
|    |    |    |    |    \-> ()S.Test
|    |    |    |    \-> S.Test
|    |    |    \-> (arg: Int)Nothing
|    |    |-- "fails" : pt=Int BYVALmode-EXPRmode (silent: value <local S> in S)
|    |    |    [search #4] start `"fails"`, searching for adaptation to pt=String("fails") => Int (silent: value <local S> in S) implicits disabled
|    |    |    [search #5] start `"fails"`, searching for adaptation to pt=(=> String("fails")) => Int (silent: value <local S> in S) implicits disabled
|    |    |    \-> <error>
|    |    no second try: (arg: Int)Nothing and arg = "fails" because error not in result: source-C:\Users\Justin\Source\SI-7809\S.scala,line-13,offset=246!=source-C:\Users\Justin\Source\SI-7809\S.scala,line-13,offset=239
\

And this is from test("works"):

|    |-- new Test().test("works") BYVALmode-EXPRmode (site: value <local S> in S)
|    |    |-- new Test().test BYVALmode-EXPRmode-FUNmode-POLYmode (silent: value <local S> in S)
|    |    |    |-- new Test() EXPRmode-POLYmode-QUALmode (silent: value <local S> in S)
|    |    |    |    |-- new Test BYVALmode-EXPRmode-FUNmode-POLYmode (silent: value <local S> in S)
|    |    |    |    |    |-- new Test EXPRmode-POLYmode-QUALmode (silent: value <local S> in S)
|    |    |    |    |    |    |-- Test FUNmode-TYPEmode (silent: value <local S> in S)
|    |    |    |    |    |    |    \-> S.Test
|    |    |    |    |    |    \-> S.Test
|    |    |    |    |    \-> ()S.Test
|    |    |    |    \-> S.Test
|    |    |    \-> (arg: Int)Nothing
|    |    |-- "works" : pt=Int BYVALmode-EXPRmode (silent: value <local S> in S)
|    |    |    [search #1] start `"works"`, searching for adaptation to pt=String("works") => Int (silent: value <local S> in S) implicits disabled
|    |    |    [search #2] start `"works"`, searching for adaptation to pt=(=> String("works")) => Int (silent: value <local S> in S) implicits disabled
|    |    |    \-> <error>
|    |    second try: (arg: Int)Nothing and "works"
|    |    [search #3] start `()S.Test`, searching for adaptation to pt=S.Test => ?{def test(x$1: ? >: String("works")): ?} (silent: value <local S> in S) implicits disabled
|    |    [search #3] considering Implicit
|    |    |-- Implicit BYVALmode-EXPRmode-FUNmode-POLYmode (silent: value <local S> in S) implicits disabled
|    |    |    \-> (t: S.Test)S.ExtClass
|    |    [adapt] Implicit adapted to (t: S.Test)S.ExtClass based on pt S.Test => ?{def test(x$1: ? >: String("works")): ?}
|    |    [search #3] success inferred value of type S.Test => ?{def test(x$1: ? >: String("works")): ?} is SearchResult(S.this.Implicit, )
|    |    |-- (t: S.Test)S.ExtClass EXPRmode-POLYmode-QUALmode (site: value <local S> in S)
|    |    |    \-> S.ExtClass
|    |    |-- S.this.Implicit(new S.this.Test()).test BYVALmode-EXPRmode-FUNmode-POLYmode (silent: value <local S> in S)
|    |    |    \-> (arg: String)Nothing
|    |    \-> Nothing

[scabug]

Justin Bailey (m4dc4p) said (edited on Apr 3, 2017 6:48:07 PM UTC):
This error is due to the existence of a AssignOrNamedArg node (https://github.com/scala/scala/blob/2.12.x/src/reflect/scala/reflect/internal/Trees.scala#L444) in the args Tree here - https://github.com/scala/scala/blob/2.12.x/src/compiler/scala/tools/nsc/typechecker/Typers.scala#L4602.

Filtering out the AssignOrNamedArg to the rhs value (i.e., the value assigned) makes the implicit resolution work, but causes other problems.

PR in progress at scala/scala#5807.

[scabug]

Justin Bailey (m4dc4p) said:
Research: Original PR for Named & Default Arguments at scala/scala@390ccac

[m4dc4p]

This bug is not a quick fix :(

[som-snytt]

Progressed to "unknown parameter name".