Improve defn.PolyFunctionOf extractor (#18442)

* Only match `RefinedType` representing the `PolyFunction`. This will
allow us to use `derivedRefinedType` on the function type.
* Only match the refinement if it is a `MethodOrPoly`. `ExprType` is not
a valid `PolyFunction` refinement.
* Remove `dealias` in `PolyFunctionOf` extractor. There was only one
case where this was necessary and it added unnecessary overhead.

Author: odersky

compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala

@@ -808,7 +808,7 @@ class CheckCaptures extends Recheck, SymTransformer:
 
         try
           val eres = expected.dealias.stripCapturing match
-            case RefinedType(_, _, rinfo: PolyType) => rinfo.resType
+            case defn.PolyFunctionOf(rinfo: PolyType) => rinfo.resType
             case expected: PolyType => expected.resType
             case _ => WildcardType
 

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

@@ -1154,11 +1154,12 @@ class Definitions {
      *
      *  Pattern: `PolyFunction { def apply: $mt }`
      */
-    def unapply(ft: Type)(using Context): Option[MethodicType] = ft.dealias match
-      case RefinedType(parent, nme.apply, mt: MethodicType)
-      if parent.derivesFrom(defn.PolyFunctionClass) =>
-        Some(mt)
-      case _ => None
+    def unapply(tpe: RefinedType)(using Context): Option[MethodOrPoly] =
+      tpe.refinedInfo match
+        case mt: MethodOrPoly
+        if tpe.refinedName == nme.apply && tpe.parent.derivesFrom(defn.PolyFunctionClass) =>
+          Some(mt)
+        case _ => None
 
     private def isValidPolyFunctionInfo(info: Type)(using Context): Boolean =
       def isValidMethodType(info: Type) = info match

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

@@ -1651,10 +1651,11 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
   def typedPolyFunctionValue(tree: untpd.PolyFunction, pt: Type)(using Context): Tree =
     val untpd.PolyFunction(tparams: List[untpd.TypeDef] @unchecked, fun) = tree: @unchecked
     val untpd.Function(vparams: List[untpd.ValDef] @unchecked, body) = fun: @unchecked
+    val dpt = pt.dealias
 
     // If the expected type is a polymorphic function with the same number of
     // type and value parameters, then infer the types of value parameters from the expected type.
-    val inferredVParams = pt match
+    val inferredVParams = dpt match
       case defn.PolyFunctionOf(poly @ PolyType(_, mt: MethodType))
       if tparams.lengthCompare(poly.paramNames) == 0 && vparams.lengthCompare(mt.paramNames) == 0 =>
         vparams.zipWithConserve(mt.paramInfos): (vparam, formal) =>
@@ -1670,7 +1671,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
       case _ =>
         vparams
 
-    val resultTpt = pt.dealias match
+    val resultTpt = dpt match
       case defn.PolyFunctionOf(poly @ PolyType(_, mt: MethodType)) =>
         untpd.InLambdaTypeTree(isResult = true, (tsyms, vsyms) =>
           mt.resultType.substParams(mt, vsyms.map(_.termRef)).substParams(poly, tsyms.map(_.typeRef)))