Scala2Unpickler: don't unpickle the same type parameter twice

Fixes #11173.
Fixes #11984.

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

@@ -434,12 +434,16 @@ class Scala2Unpickler(bytes: Array[Byte], classRoot: ClassDenotation, moduleClas
     var name = at(nameref, () => readName()(using ctx))
     val owner = readSymbolRef()
 
-    if (name eq nme.getClass_) && defn.hasProblematicGetClass(owner.name) then
+    var flags = unpickleScalaFlags(readLongNat(), name.isTypeName)
+
+    if (name eq nme.getClass_) && defn.hasProblematicGetClass(owner.name)
+       // Scala 2 sometimes pickle the same type parameter symbol multiple times
+       // (see i11173 for an example), but we should only unpickle it once.
+       || tag == TYPEsym && flags.is(TypeParam) && symScope(owner).lookup(name.asTypeName).exists
+    then
       // skip this member
       return NoSymbol
 
-    var flags = unpickleScalaFlags(readLongNat(), name.isTypeName)
-
     name = name.adjustIfModuleClass(flags)
     if (flags.is(Method))
       name =
@@ -1333,4 +1337,4 @@ class Scala2Unpickler(bytes: Array[Byte], classRoot: ClassDenotation, moduleClas
       case other =>
         errorBadSignature("expected an TypeDef (" + other + ")")
     }
-}
+}

sbt-dotty/sbt-test/scala2-compat/i11173/app/App.scala

@@ -0,0 +1,3 @@
+import i11173.Bar
+
+def test(x: Bar[_]): Unit = ()

sbt-dotty/sbt-test/scala2-compat/i11173/build.sbt

@@ -0,0 +1,13 @@
+val scala3Version = sys.props("plugin.scalaVersion")
+val scala2Version = "2.13.5"
+
+lazy val lib = project.in(file("lib"))
+  .settings(
+    scalaVersion := scala2Version
+  )
+
+lazy val app = project.in(file("app"))
+  .dependsOn(lib)
+  .settings(
+    scalaVersion := scala3Version
+  )

sbt-dotty/sbt-test/scala2-compat/i11173/lib/Lib.scala

@@ -0,0 +1,14 @@
+
+// Compile this with Scala 2.13
+package i11173
+
+trait DU[A, B]
+trait H[F[_]]
+
+trait Foo[E] {
+  def foo: H[({type L[A] = DU[E, A]})#L]
+}
+
+trait Bar[E] extends Foo[E] {
+  def bar = foo   // important note: return type not specified
+}

sbt-dotty/sbt-test/scala2-compat/i11173/project/plugins.sbt

@@ -0,0 +1 @@
+addSbtPlugin("ch.epfl.lamp" % "sbt-dotty" % sys.props("plugin.version"))

sbt-dotty/sbt-test/scala2-compat/i11173/test

@@ -0,0 +1 @@
+> app/compile