Merge pull request #3727 from dotty-staging/fi-#3542

Fix #3542: Avoid comparisons between method types and typevars

Author: odersky

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

@@ -1106,7 +1106,7 @@ class Namer { typer: Typer =>
         mdef match {
           case mdef: DefDef if mdef.name == nme.ANON_FUN =>
             val hygienicType = avoid(rhsType, paramss.flatten)
-            if (!(hygienicType <:< tpt.tpe))
+            if (!hygienicType.isValueType || !(hygienicType <:< tpt.tpe))
               ctx.error(i"return type ${tpt.tpe} of lambda cannot be made hygienic;\n" +
                 i"it is not a supertype of the hygienic type $hygienicType", mdef.pos)
             //println(i"lifting $rhsType over $paramss -> $hygienicType = ${tpt.tpe}")

tests/neg/i3542a.scala

@@ -0,0 +1,5 @@
+object Foo {
+  def f()()(implicit ctx: String): Int = ???
+  def at[T](op: () => T): Unit = ()
+  at(() => f()) // error: missing ()
+}