Fix #1037

Achieved by tweaking from where we get the parameter types of an eta-expansion.

Author: odersky

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

@@ -1460,12 +1460,16 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
           }
       case wtp: MethodType if !pt.isInstanceOf[SingletonType] =>
         val arity =
-          if (defn.isFunctionType(pt)) defn.functionArity(pt)
+          if (defn.isFunctionType(pt))
+            if (!isFullyDefined(pt, ForceDegree.none) && isFullyDefined(wtp, ForceDegree.none))
+              // prioritize method parameter types over expected types as parameter types of the eta-expanded closure
+              0
+            else defn.functionArity(pt)
           else if (pt eq AnyFunctionProto) wtp.paramTypes.length
           else -1
-        if (arity >= 0 && !tree.symbol.isConstructor)
+        if (arity >= 0 && !tree.symbol.isConstructor) {
           typed(etaExpand(tree, wtp, arity), pt)
-        else if (wtp.paramTypes.isEmpty)
+        } else if (wtp.paramTypes.isEmpty)
           adaptInterpolated(tpd.Apply(tree, Nil), pt, EmptyTree)
         else if (wtp.isImplicit)
           err.typeMismatch(tree, pt)

tests/pos/i1036.scala

@@ -0,0 +1,10 @@
+object Test {
+  def second[sA, sB <: sA](foo: sA, bar: sB): sB = bar
+  def expectString(s: String) = s
+
+  def test = {
+    val x = second(Set.empty[String], Set.empty)
+    x map expectString
+    second(Set.empty[String], Set.empty) map expectString
+  }
+}