compilation failures on tests in Scala 2.13 community build

[SethTisue]

from https://scala-ci.typesafe.com/view/scala-2.13.x/job/scala-2.13.x-integrate-community-build/905/consoleFull :

[scala-java8-compat] [error] /home/jenkins/workspace/scala-2.13.x-integrate-community-build/target-0.9.9/project-builds/scala-java8-compat-3b0fd4decb75f5e349e7c755590b4855660ac3b4/src/test/scala/scala/compat/java8/StreamConvertersTest.scala:249: type mismatch;
[scala-java8-compat] [error]  found   : java.util.stream.Stream[Short]
[scala-java8-compat] [error]  required: java.util.stream.IntStream
[scala-java8-compat] [error]     assertEquals(Vector[Short](1.toShort, 2.toShort, 3.toShort), (Array[Short](1.toShort, 2.toShort, 3.toShort).seqStream: IntStream).toScala[Vector])
[scala-java8-compat] [error]                                                                                                                 ^
[scala-java8-compat] [error] /home/jenkins/workspace/scala-2.13.x-integrate-community-build/target-0.9.9/project-builds/scala-java8-compat-3b0fd4decb75f5e349e7c755590b4855660ac3b4/src/test/scala/scala/compat/java8/StreamConvertersTest.scala:253: type mismatch;
[scala-java8-compat] [error]  found   : java.util.stream.Stream[Int]
[scala-java8-compat] [error]  required: java.util.stream.IntStream
[scala-java8-compat] [error]     assertEquals(Vector[Int](1, 2, 3), (Vector[Int](1, 2, 3).seqStream: IntStream).toScala[Vector])
[scala-java8-compat] [error]                                                              ^
[scala-java8-compat] [error] /home/jenkins/workspace/scala-2.13.x-integrate-community-build/target-0.9.9/project-builds/scala-java8-compat-3b0fd4decb75f5e349e7c755590b4855660ac3b4/src/test/scala/scala/compat/java8/StreamConvertersTest.scala:254: type mismatch;
[scala-java8-compat] [error]  found   : java.util.stream.Stream[Short]
[scala-java8-compat] [error]  required: java.util.stream.IntStream
[scala-java8-compat] [error]     assertEquals(Vector[Short](1.toShort, 2.toShort, 3.toShort), (Vector[Short](1.toShort, 2.toShort, 3.toShort).seqStream: IntStream).toScala[Vector])
[scala-java8-compat] [error]                                                                                                                  ^
[scala-java8-compat] [error] /home/jenkins/workspace/scala-2.13.x-integrate-community-build/target-0.9.9/project-builds/scala-java8-compat-3b0fd4decb75f5e349e7c755590b4855660ac3b4/src/test/scala/scala/compat/java8/StreamConvertersTest.scala:269: type mismatch;
[scala-java8-compat] [error]  found   : java.util.stream.Stream[Int]
[scala-java8-compat] [error]  required: java.util.stream.IntStream
[scala-java8-compat] [error]     val ss2: IntStream = ss
[scala-java8-compat] [error]                          ^
[scala-java8-compat] [error] four errors found
[scala-java8-compat] [error] (root/test:compileIncremental) Compilation failed

[retronym]

I can reproduce locally:

⚡ VER=$(scala-ref-version heads/2.13.x); sbt -Dnodocs=true ++$VER 'set scalacOptions += "-nowarn"' test:compile
[info] Loading global plugins from /Users/jz/.sbt/0.13/plugins
[info] Loading project definition from /Users/jz/code/scala-java8-compat/project/project
[info] Loading project definition from /Users/jz/code/scala-java8-compat/project
[info] Set current project to scala-java8-compat (in build file:/Users/jz/code/scala-java8-compat/)
[info] Setting version to 2.13.0-pre-4503ad2-SNAPSHOT
[info] Reapplying settings...
[info] Set current project to scala-java8-compat (in build file:/Users/jz/code/scala-java8-compat/)
[info] Defining root/*:scalacOptions
[info] The new value will be used by root/compile:scalacOptions, root/genjavadoc:scalacOptions
[info] Reapplying settings...
[info] Set current project to scala-java8-compat (in build file:/Users/jz/code/scala-java8-compat/)
[info] Running WrapFnGen /Users/jz/code/scala-java8-compat/target/scala-2.13.0-pre-4503ad2-SNAPSHOT/src_managed/main/FunctionConverters.scala
[info] Compiling 40 Scala sources and 52 Java sources to /Users/jz/code/scala-java8-compat/target/scala-2.13.0-pre-4503ad2-SNAPSHOT/classes...
[info] /Users/jz/code/scala-java8-compat/src/main/java/scala/compat/java8/ScalaStreamSupport.java: Some input files use unchecked or unsafe operations.
[info] /Users/jz/code/scala-java8-compat/src/main/java/scala/compat/java8/ScalaStreamSupport.java: Recompile with -Xlint:unchecked for details.
[info] Compiling 6 Scala sources and 7 Java sources to /Users/jz/code/scala-java8-compat/target/scala-2.13.0-pre-4503ad2-SNAPSHOT/test-classes...
[error] /Users/jz/code/scala-java8-compat/src/test/scala/scala/compat/java8/StreamConvertersTest.scala:249: type mismatch;
[error]  found   : java.util.stream.Stream[Short]
[error]  required: java.util.stream.IntStream
[error]     assertEquals(Vector[Short](1.toShort, 2.toShort, 3.toShort), (Array[Short](1.toShort, 2.toShort, 3.toShort).seqStream: IntStream).toScala[Vector])
[error]                                                                                                                 ^
[error] /Users/jz/code/scala-java8-compat/src/test/scala/scala/compat/java8/StreamConvertersTest.scala:253: type mismatch;
[error]  found   : java.util.stream.Stream[Int]
[error]  required: java.util.stream.IntStream
[error]     assertEquals(Vector[Int](1, 2, 3), (Vector[Int](1, 2, 3).seqStream: IntStream).toScala[Vector])
[error]                                                              ^
[error] /Users/jz/code/scala-java8-compat/src/test/scala/scala/compat/java8/StreamConvertersTest.scala:254: type mismatch;
[error]  found   : java.util.stream.Stream[Short]
[error]  required: java.util.stream.IntStream
[error]     assertEquals(Vector[Short](1.toShort, 2.toShort, 3.toShort), (Vector[Short](1.toShort, 2.toShort, 3.toShort).seqStream: IntStream).toScala[Vector])
[error]                                                                                                                  ^
[error] /Users/jz/code/scala-java8-compat/src/test/scala/scala/compat/java8/StreamConvertersTest.scala:269: type mismatch;
[error]  found   : java.util.stream.Stream[Int]
[error]  required: java.util.stream.IntStream
[error]     val ss2: IntStream = ss
[error]                          ^
[error] four errors found
[error] (root/test:compileIncremental) Compilation failed
[error] Total time: 18 s, completed 25/01/2018 12:31:25 PM

[retronym]

I've partially minimized

--- /tmp/okay	2018-01-25 13:06:10.000000000 +1000
+++ /tmp/broke	2018-01-25 13:06:26.000000000 +1000
@@ -1,48 +1,58 @@
 package scala.compat.java8

 import java.util.stream._
 import StreamConverters._

 class Test {
-  x
   y
+  x

   def x = EnrichAnySteppableWithSeqStream(Vector[Int](1, 2, 3)).seqStream: IntStream
   def y =  EnrichAnySteppableWithSeqStream(Array[String]("a", "b")).seqStream: Stream[String]

 }
 /Users/jz/code/scala-java8-compat/src/test/scala/scala/compat/java8/StreamConvertersTest.scala:10: $conforms is not a valid implicit value for scala.collection.immutable.Vector[Int] => scala.compat.java8.converterImpl.MakesStepper[A,Any] because:
-hasMatchingSymbol reported error: type mismatch;
- found   : scala.collection.immutable.Vector[Int] <:< scala.collection.immutable.Vector[Int]
- required: scala.collection.immutable.Vector[Int] => scala.compat.java8.converterImpl.MakesStepper[?,Any]
-  def x = EnrichAnySteppableWithSeqStream(Vector[Int](1, 2, 3)).seqStream: IntStream
-                                         ^
-/Users/jz/code/scala-java8-compat/src/test/scala/scala/compat/java8/StreamConvertersTest.scala:11: materializing requested scala.reflect.type.ClassTag[String] using scala.reflect.`package`.materializeClassTag[String]()
 /Users/jz/code/scala-java8-compat/src/test/scala/scala/compat/java8/StreamConvertersTest.scala:11: materializing requested scala.reflect.type.ClassTag[String] using scala.reflect.`package`.materializeClassTag[String]()
   def y =  EnrichAnySteppableWithSeqStream(Array[String]("a", "b")).seqStream: Stream[String]
                                                         ^
 /Users/jz/code/scala-java8-compat/src/test/scala/scala/compat/java8/StreamConvertersTest.scala:11: $conforms is not a valid implicit value for Array[String] => scala.compat.java8.converterImpl.MakesStepper[A,Any] because:
 hasMatchingSymbol reported error: type mismatch;
  found   : Array[String] <:< Array[String]
  required: Array[String] => scala.compat.java8.converterImpl.MakesStepper[?,Any]
   def y =  EnrichAnySteppableWithSeqStream(Array[String]("a", "b")).seqStream: Stream[String]
                                           ^
+/Users/jz/code/scala-java8-compat/src/test/scala/scala/compat/java8/StreamConvertersTest.scala:10: $conforms is not a valid implicit value for scala.collection.immutable.Vector[Int] => scala.compat.java8.converterImpl.MakesStepper[A,Any] because:
+hasMatchingSymbol reported error: type mismatch;
+ found   : scala.collection.immutable.Vector[Int] <:< scala.collection.immutable.Vector[Int]
+ required: scala.collection.immutable.Vector[Int] => scala.compat.java8.converterImpl.MakesStepper[?,Any]
+  def x = EnrichAnySteppableWithSeqStream(Vector[Int](1, 2, 3)).seqStream: IntStream
+                                         ^
+/Users/jz/code/scala-java8-compat/src/test/scala/scala/compat/java8/StreamConvertersTest.scala:10: java8.this.StreamShape.IntValue is not a valid implicit value for scala.compat.java8.StreamShape[Int,S] because:
+type parameters weren't correctly instantiated outside of the implicit tree: inferred type arguments [java.util.stream.IntStream] do not conform to method EnrichAnySteppableWithSeqStream's type parameter bounds [S <: java.util.stream.BaseStream[_, java.util.stream.Stream[Int]]]
+  def x = EnrichAnySteppableWithSeqStream(Vector[Int](1, 2, 3)).seqStream: IntStream
+                                         ^
+/Users/jz/code/scala-java8-compat/src/test/scala/scala/compat/java8/StreamConvertersTest.scala:10: error: type mismatch;
+ found   : java.util.stream.Stream[Int]
+ required: java.util.stream.IntStream
+  def x = EnrichAnySteppableWithSeqStream(Vector[Int](1, 2, 3)).seqStream: IntStream
+                                                                ^
 [[syntax trees at end of                     typer]] // StreamConvertersTest.scala
 package scala.compat.java8 {
   import java.util.stream._;
   import StreamConverters._;
   class Test extends scala.AnyRef {
     def <init>(): scala.compat.java8.Test = {
       Test.super.<init>();
       ()
     };
-    Test.this.x;
     Test.this.y;
-    def x: java.util.stream.IntStream = (StreamConverters.EnrichAnySteppableWithSeqStream[Int, java.util.stream.IntStream, scala.collection.immutable.Vector[Int]](scala.`package`.Vector.apply[Int](1, 2, 3))({
+    Test.this.x;
+    def x: java.util.stream.IntStream = (StreamConverters.EnrichAnySteppableWithSeqStream[Int, java.util.stream.Stream[Int], scala.collection.immutable.Vector[Int]](scala.`package`.Vector.apply[Int](1, 2, 3))({
   ((underlying: Vector[Int]) => StreamConverters.richVectorCanStep[Int](underlying))
-}, java8.this.StreamShape.IntValue).seqStream: java.util.stream.IntStream);
+}, java8.this.StreamShape.anyStreamShape[Int]).<seqStream: error>: java.util.stream.IntStream);
     def y: java.util.stream.Stream[String] = (StreamConverters.EnrichAnySteppableWithSeqStream[String, java.util.stream.Stream[String], Array[String]](scala.Array.apply[String]("a", "b")((ClassTag.apply[String](classOf[java.lang.String]): scala.reflect.ClassTag[String])))({
   ((underlying: Array[String]) => StreamConverters.richArrayCanStep[String](underlying))
 }, java8.this.StreamShape.anyStreamShape[String]).seqStream: java.util.stream.Stream[String])
   }
 }

+one error found

[retronym]

Standalone:

package scala.compat.java8

trait BaseStream[T, S <: BaseStream[T, S]]
trait Stream[T] extends BaseStream[T, Stream[T]]
trait IntStream extends BaseStream[Integer, IntStream]

sealed trait SS[T, S <: BaseStream[_, S]]
object SSImplicits extends Low {
  implicit val IntValue: SS[Int, IntStream] = null
}
trait Low {
  implicit def anyStreamShape[T]: SS[T, Stream[T]] = null
}

import SSImplicits.{IntValue, anyStreamShape}

class Test {
  implicit def f[A, S <: BaseStream[_, S], CC](a: A)(implicit ss: SS[A, S]): S = ???

  // switch these lines and typechecking the body of `def x` fails in 2.13.x
  x
  y

  def x = f(0): IntStream
  def y = f[String, Stream[String], Vector[String]]("")

}

[retronym]

Regressed in scala/scala#6140

[milessabin]

Hopefully fixed in scala/scala#6308.