Fix handling of by-name default getters

There was an omission before, but as long as  byname parameters were ExprTypes it did
not lead to problems.

Author: odersky

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

@@ -1656,7 +1656,7 @@ class Namer { typer: Typer =>
             ctx.defContext(sym).denotNamed(original)
         def paramProto(paramss: List[List[Type]], idx: Int): Type = paramss match {
           case params :: paramss1 =>
-            if (idx < params.length) params(idx)
+            if (idx < params.length) params(idx).widenByName
             else paramProto(paramss1, idx - params.length)
           case nil =>
             NoType

tests/pos/byname-default.scala

@@ -0,0 +1,5 @@
+// Extraction of a failing case in scalaSTM
+object Test:
+
+  def apply[A](init: => A = null.asInstanceOf[A]) = init
+