Fix #6891: Assertion failed when naming parameter in polymorphic function type

The reason behind the failure was an incorrect desugaring of
polymorphic functions. `[A] => A => A` desugars to, essentially,
`def apply[A](x$1: A): A`. Whereas `[A] => (f: A) => A` desugars
to `def apply[A](x$1: f: A): A`, which of course does not make
sense.

Author: anatoliykmetyuk

compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -1500,8 +1500,9 @@ object desugar {
           // Desugar [T_1, ..., T_M] -> (P_1, ..., P_N) => R
           // Into    scala.PolyFunction { def apply[T_1, ..., T_M](x$1: P_1, ..., x$N: P_N): R }
 
-          val applyVParams = vargs.zipWithIndex.map { case (p, n) =>
-            makeSyntheticParameter(n + 1, p).withAddedFlags(mods.flags)
+          val applyVParams = vargs.zipWithIndex.map {
+            case (p: ValDef, _) => p.withAddedFlags(mods.flags)
+            case (p, n) => makeSyntheticParameter(n + 1, p).withAddedFlags(mods.flags)
           }
           RefinedTypeTree(polyFunctionTpt, List(
             DefDef(nme.apply, applyTParams, List(applyVParams), res, EmptyTree)

tests/neg/i6891.scala

@@ -0,0 +1,10 @@
+object Test {
+  trait Foo { type X }
+  class Bar extends Foo { type X = Int }
+  type F = [A <: Foo] => (f: A) => f.X => f.X
+
+  def main(args: Array[String]): Unit = {
+    val g: F = [A <: Foo] => (f: A) => (x: f.X) => x
+    println(g(new Bar)("foo")) // error
+  }
+}

tests/pos/i6891.scala

@@ -0,0 +1 @@
+def foo(App: [A] => (f: A) => Any): Any = ???

tests/run/i6891.check

@@ -0,0 +1 @@
+1

tests/run/i6891.scala

@@ -0,0 +1,10 @@
+object Test {
+  trait Foo { type X }
+  class Bar extends Foo { type X = Int }
+  type F = [A <: Foo] => (f: A) => f.X => f.X
+
+  def main(args: Array[String]): Unit = {
+    val g: F = [A <: Foo] => (f: A) => (x: f.X) => x
+    println(g(new Bar)(1))
+  }
+}