Specialize byname functions

I tried to extend specialization to all context functions, not just ones of 0 arity.
But that runs into problems for dependent context functions, since the necessary casts
get complicated. Since context functions over primitive types are an anti-pattern anyway
I don't think we need to optimize this case, after all.

Author: odersky

compiler/src/dotty/tools/dotc/core/Definitions.scala

@@ -1546,7 +1546,8 @@ class Definitions {
     new PerRun(Function2SpecializedReturnTypes.map(_.symbol))
 
   def isSpecializableFunction(cls: ClassSymbol, paramTypes: List[Type], retType: Type)(using Context): Boolean =
-    paramTypes.length <= 2 && cls.derivesFrom(FunctionClass(paramTypes.length))
+    paramTypes.length <= 2
+    && (cls.derivesFrom(FunctionClass(paramTypes.length)) || isByNameClass(cls))
     && isSpecializableFunctionSAM(paramTypes, retType)
 
   /** If the Single Abstract Method of a Function class has this type, is it specializable? */

compiler/src/dotty/tools/dotc/transform/SpecializeFunctions.scala

@@ -80,7 +80,15 @@ class SpecializeFunctions extends MiniPhase {
           val specializedApply = nme.apply.specializedFunction(retType, argTypes)
           val newSel = fun match
             case Select(qual, _) =>
-              qual.select(specializedApply)
+              val qual1 = qual.tpe.widen match
+                case ByNameType(res) =>
+                  // Need to cast to regular function, since specialied apply methods
+                  // are not members of ContextFunction0. The cast will be eliminated in
+                  // erasure.
+                  qual.cast(defn.FunctionOf(Nil, res))
+                case _ =>
+                  qual
+              qual1.select(specializedApply)
             case _ =>
               (fun.tpe: @unchecked) match
                 case TermRef(prefix: ThisType, name) =>