Fix #3901: Survive parameter dependencies on by-name parameters

Author: odersky

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

@@ -418,7 +418,7 @@ object ProtoTypes {
 
   /** Create a new TypeVar that represents a dependent method parameter singleton */
   def newDepTypeVar(tp: Type)(implicit ctx: Context): TypeVar =
-    newTypeVar(TypeBounds.upper(AndType(tp, defn.SingletonClass.typeRef)))
+    newTypeVar(TypeBounds.upper(AndType(tp.widenExpr, defn.SingletonClass.typeRef)))
 
   /** The result type of `mt`, where all references to parameters of `mt` are
    *  replaced by either wildcards (if typevarsMissContext) or TypeParamRefs.

tests/neg/i3901.scala

@@ -0,0 +1,4 @@
+object Crash {
+  def f(cond: => Boolean): cond.type = ???
+  f(true)
+}