Don't lose paramSyms when unpickling from Scala 2

We could lose them for curried methods since the nested method type was copied
in case there was a depencency on a parameter of the first method type.

Fixes scala#13238

Author: odersky

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 =>
+            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 =>

tests/neg/i13238/boom_2.scala

@@ -0,0 +1,2 @@
+def boom(foo: Foo) = foo.foo(foo)  // error
+

tests/neg/i13238/foo_1.scala

@@ -0,0 +1,5 @@
+// Should be compiled with 2.13
+trait Bar
+class Foo {
+  def foo(out: Foo)(implicit bar: Bar): out.type = out
+}