Don't lose paramSyms when unpickling from Scala 2

compiler/src/dotty/tools/dotc/core/unpickleScala2/Scala2Unpickler.scala

@@ -847,6 +847,12 @@ class Scala2Unpickler(bytes: Array[Byte], classRoot: ClassDenotation, moduleClas
         val maker = MethodType.companion(
           isImplicit = tag == IMPLICITMETHODtpe || params.nonEmpty && params.head.is(Implicit))
         val result = maker.fromSymbols(params, restpe)
+        result.resType match
+          case restpe1: MethodType if restpe1 ne restpe =>
+            val prevResParams = paramsOfMethodType.remove(restpe)
+            if prevResParams != null then
+              paramsOfMethodType.put(restpe1, prevResParams)
+          case _ =>
         if params.nonEmpty then paramsOfMethodType.put(result, params)
         result
       case POLYtpe =>

sbt-test/compilerReporter/i7442/project/Reporter.scala

@@ -24,7 +24,7 @@ object Reporter {
 
   lazy val checkSettings = Seq(
     Compile / compile / compilerReporter := reporter,
-    check := (compile in Compile).failure.map(_ => {
+    check := (Compile / compile).failure.map(_ => {
       println(reporter.problems.toList)
       assert(reporter.problems.length == 1)
       val problem = reporter.problems.head

sbt-test/compilerReporter/simple/project/Reporter.scala

@@ -24,7 +24,7 @@ object Reporter {
 
   lazy val checkSettings = Seq(
     Compile / compile / compilerReporter := reporter,
-    check := (compile in Compile).failure.map(_ => {
+    check := (Compile / compile).failure.map(_ => {
       val problems = reporter.problems
       println(problems.toList)
       assert(problems.size == 1)

sbt-test/scala2-compat/i13238/app/App.scala

@@ -0,0 +1,5 @@
+package app
+
+import lib.*
+
+def boom(foo: Foo) = foo.foo(foo) // error: no implicit argument of type lib.Bar was found for parameter bar of method foo in class Foo

sbt-test/scala2-compat/i13238/build.sbt

@@ -0,0 +1,14 @@
+val scala3Version = sys.props("plugin.scalaVersion")
+val scala2Version = sys.props("plugin.scala2Version")
+
+lazy val lib = project.in(file("lib"))
+  .settings(
+    scalaVersion := scala2Version
+  )
+
+lazy val app = project.in(file("app"))
+  .dependsOn(lib)
+  .settings(Reporter.checkSettings)
+  .settings(
+    scalaVersion := scala3Version
+  )

sbt-test/scala2-compat/i13238/lib/Lib.scala

@@ -0,0 +1,6 @@
+package lib
+
+trait Bar
+class Foo {
+  def foo(out: Foo)(implicit bar: Bar): out.type = out
+}

sbt-test/scala2-compat/i13238/project/Reporter.scala

@@ -0,0 +1,33 @@
+import sbt._
+import Keys._
+import KeyRanks.DTask
+
+object Reporter {
+  import xsbti.{Reporter, Problem, Position, Severity}
+
+  lazy val check = TaskKey[Unit]("check", "Check compilation output")
+
+  // compilerReporter is marked private in sbt
+  lazy val compilerReporter = TaskKey[xsbti.Reporter]("compilerReporter", "Experimental hook to listen (or send) compilation failure messages.", DTask)
+
+  lazy val reporter =
+    new xsbti.Reporter {
+      private val buffer = collection.mutable.ArrayBuffer.empty[Problem]
+      def reset(): Unit = buffer.clear()
+      def hasErrors: Boolean = buffer.exists(_.severity == Severity.Error)
+      def hasWarnings: Boolean = buffer.exists(_.severity == Severity.Warn)
+      def printSummary(): Unit = println(problems.mkString(System.lineSeparator))
+      def problems: Array[Problem] = buffer.toArray
+      def log(problem: Problem): Unit = buffer.append(problem)
+      def comment(pos: xsbti.Position, msg: String): Unit = ()
+    }
+
+  lazy val checkSettings = Seq(
+    Compile / compile / compilerReporter := reporter,
+    check := (Compile / compile).failure.map(_ => {
+      val problems = reporter.problems
+      assert(problems.size == 1, problems.size)
+      assert(problems.head.position.line.get() == 5, problems.head.position.line)
+    }).value
+  )
+}

sbt-test/scala2-compat/i13238/test

@@ -0,0 +1,2 @@
+> lib/compile
+> app/check