Handle dependent context functions

Add `FunctionTypeOfMethod` extractor that matches any kind of function
and return its method type.

We use this extractor instead of `ContextFunctionType` to all of
 * `ContextFunctionN[...]`
 * `ContextFunctionN[...] { def apply(using ...): R }` where `R` might
    be dependent on the parameters.
 * `PolyFunction { def apply(using ...): R }` where `R` might
    be dependent on the parameters. Currently this one would have at
    least one erased parameter.

compiler/src/dotty/tools/dotc/ast/TreeInfo.scala

@@ -954,7 +954,7 @@ trait TypedTreeInfo extends TreeInfo[Type] { self: Trees.Instance[Type] =>
   def isStructuralTermSelectOrApply(tree: Tree)(using Context): Boolean = {
     def isStructuralTermSelect(tree: Select) =
       def hasRefinement(qualtpe: Type): Boolean = qualtpe.dealias match
-        case defn.PolyFunctionOf(_) =>
+        case defn.FunctionTypeOfMethod(_) =>
           false
         case RefinedType(parent, rname, rinfo) =>
           rname == tree.name || hasRefinement(parent)

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

@@ -1108,6 +1108,22 @@ class Definitions {
     //  - .linkedClass: the ClassSymbol of the enumeration (class E)
     sym.owner.linkedClass.typeRef
 
+  object FunctionTypeOfMethod {
+    /** Matches a `FunctionN[...]`/`ContextFunctionN[...]` or refined `PolyFunction`/`FunctionN[...]`/`ContextFunctionN[...]`.
+     *  Extracts the method type type and apply info.
+     */
+    def unapply(ft: Type)(using Context): Option[MethodOrPoly] = {
+      ft match
+        case RefinedType(parent, nme.apply, mt: MethodOrPoly)
+        if parent.derivesFrom(defn.PolyFunctionClass) || isFunctionNType(parent) =>
+          Some(mt)
+        case FunctionOf(argTypes, resultType, isContextual) =>
+          val methodType = if isContextual then ContextualMethodType else MethodType
+          Some(methodType(argTypes, resultType))
+        case _ => None
+    }
+  }
+
   object FunctionOf {
     def apply(args: List[Type], resultType: Type, isContextual: Boolean = false)(using Context): Type =
       val mt = MethodType.companion(isContextual, false)(args, resultType)

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

@@ -58,7 +58,8 @@ object ContextFunctionResults:
    */
   def contextResultsAreErased(sym: Symbol)(using Context): Boolean =
     def allErased(tp: Type): Boolean = tp.dealias match
-      case defn.ContextFunctionType(_, resTpe, erasedParams) => !erasedParams.contains(false) && allErased(resTpe)
+      case ft @ defn.FunctionTypeOfMethod(mt: MethodType) if mt.isContextualMethod =>
+        !mt.erasedParams.contains(false) && allErased(mt.resType)
       case _ => true
     contextResultCount(sym) > 0 && allErased(sym.info.finalResultType)
 
@@ -72,8 +73,8 @@ object ContextFunctionResults:
         integrateContextResults(rt, crCount)
       case tp: MethodOrPoly =>
         tp.derivedLambdaType(resType = integrateContextResults(tp.resType, crCount))
-      case defn.ContextFunctionType(argTypes, resType, erasedParams) =>
-        MethodType(argTypes, integrateContextResults(resType, crCount - 1))
+      case defn.FunctionTypeOfMethod(mt) if mt.isContextualMethod =>
+        mt.derivedLambdaType(resType = integrateContextResults(mt.resType, crCount - 1))
 
   /** The total number of parameters of method `sym`, not counting
    *  erased parameters, but including context result parameters.
@@ -103,7 +104,7 @@ object ContextFunctionResults:
     def recur(tp: Type, n: Int): Type =
       if n == 0 then tp
       else tp match
-        case defn.ContextFunctionType(_, resTpe, _) => recur(resTpe, n - 1)
+        case defn.FunctionTypeOfMethod(mt) => recur(mt.resType, n - 1)
     recur(meth.info.finalResultType, depth)
 
   /** Should selection `tree` be eliminated since it refers to an `apply`
@@ -117,8 +118,8 @@ object ContextFunctionResults:
     else tree match
       case Select(qual, name) =>
         if name == nme.apply then
-          qual.tpe match
-            case defn.ContextFunctionType(_, _, _) =>
+          qual.tpe.nn.dealias match
+            case defn.FunctionTypeOfMethod(mt) if mt.isContextualMethod =>
               integrateSelect(qual, n + 1)
             case _ if defn.isContextFunctionClass(tree.symbol.maybeOwner) => // for TermRefs
               integrateSelect(qual, n + 1)