Fix #8861: Replace type variables by wildcards in closure results

When passing down an expected result type for typing a closure body,
replace any type variables by wildcards. This is needed to avoid such
type variables getting constraints that are polluted with local parameters.

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

@@ -1496,7 +1496,14 @@ class Namer { typer: Typer =>
       case TypedSplice(tpt: TypeTree) if !isFullyDefined(tpt.tpe, ForceDegree.none) =>
         mdef match {
           case mdef: DefDef if mdef.name == nme.ANON_FUN =>
-            val rhsType = typedAheadExpr(mdef.rhs, tpt.tpe).tpe
+            // Need to constrain the type varianbes in the handed-down expected
+            // result type with the actual result type of the closure body.
+            // Two steps:
+            //  1. Type closure body with the wildcard approximation of the expected result type.
+            //     wildApprox is needed since otherwise type variables in the expected result type
+            //     might get polluted constraints referring to local parameters. See run/i8861.scala.
+            //  2. Constrain the expected result type from below with the actual result type.
+            val rhsType = typedAheadExpr(mdef.rhs, wildApprox(tpt.tpe)).tpe
             val hygienicType = avoid(rhsType, paramss.flatten)
             if (!hygienicType.isValueType || !(hygienicType <:< tpt.tpe))
               ctx.error(i"return type ${tpt.tpe} of lambda cannot be made hygienic;\n" +

tests/neg/i6565.scala → tests/pos/i6565.scala

@@ -12,6 +12,6 @@ lazy val ok: Lifted[String] = { // ok despite map returning a union
   point("a").map(_ => if true then "foo" else error) // ok
 }
 
-lazy val bad: Lifted[String] = { // found Lifted[Object]
-  point("a").flatMap(_ => point("b").map(_ => if true then "foo" else error)) // error
+lazy val nowAlsoOK: Lifted[String] = {
+  point("a").flatMap(_ => point("b").map(_ => if true then "foo" else error)) // now also OK
 }