Always do eta expansion if arity >= 1

Implements another part of #2570

Author: odersky

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -2010,8 +2010,12 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
               // prioritize method parameter types as parameter types of the eta-expanded closure
               0
             else defn.functionArity(ptNorm)
-          else if (pt eq AnyFunctionProto) wtp.paramInfos.length
-          else -1
+          else {
+            val nparams = wtp.paramInfos.length
+            if (nparams > 0 || pt.eq(AnyFunctionProto)) nparams
+            else -1 // no eta expansion in this case
+          }
+
         if (arity >= 0 && !tree.symbol.isConstructor)
           typed(etaExpand(tree, wtp, arity), pt)
         else if (wtp.paramInfos.isEmpty)

tests/pos/i2570.scala

@@ -21,9 +21,18 @@ object Test {
   def sum2(x: Int, ys: Int*) = (x /: ys)(_ + _)
   val h1: ((Int, Seq[Int]) => Int) = sum2
 
-// Not yet:
-//  val h1 = repeat
-//  val h2: (String, Int, Int) = h1
-//  val h3 = sum
-//  val h4: (Int, => Int) = h3
+  val h3 = repeat
+  val h4: ((String, Int, Int) => String) = h3
+  val h5 = sum
+  val h6: ((Int, => Int) => Int) = h5
+
+  class A
+  class B
+  class C
+  implicit object b extends B
+
+  def foo(x: A)(implicit bla: B): C = ???
+
+  val h7 = foo
+  val h8: A => C = h7
 }