Refactor typedSelect

The aim of the refactoring is to be clearer and to be able to drop ExtMethodApply.

Two main strategies:

 1. Move some code out of adapt and assignType into typedSelect.
 2. Concentrate extension method expansion and conversions for selections in a
    method `tryExtensionOrConversion. The method is called from `typedSelect`
    and `tryInsertImplicitOnQualifier`.

The aim of the refactoring is to move all select-dependent adaptations and re-tries into
typedSelect. Right now some are in adapt and some are in assignType. This is awkward
since it means that

 - we do too much in TypeAssigner. Sine it aims to be minimal TypeAssigner should have no business doing adaptations
and retries.
 - we do some adaptations too early in adpt. This means we need the kludge of wrapping
trees in ExtMethodApply which causes #8182, among others.

Fixes #8182

Author: odersky

community-build/community-projects/munit

@@ -645,8 +645,6 @@ class RefinedPrinter(_ctx: Context) extends PlainPrinter(_ctx) {
         keywordStr("${") ~ toTextGlobal(dropBlock(tree)) ~ keywordStr("}")
       case TypSplice(tree) =>
         keywordStr("${") ~ toTextGlobal(dropBlock(tree)) ~ keywordStr("}")
-      case tree: Applications.ExtMethodApply =>
-        toText(tree.app) ~ Str("(ext method apply)").provided(printDebug)
       case Thicket(trees) =>
         "Thicket {" ~~ toTextGlobal(trees, "\n") ~~ "}"
       case MacroTree(call) =>

compiler/src/dotty/tools/dotc/printing/RefinedPrinter.scala

@@ -195,23 +195,6 @@ object Applications {
 
   def wrapDefs(defs: mutable.ListBuffer[Tree], tree: Tree)(using Context): Tree =
     if (defs != null && defs.nonEmpty) tpd.Block(defs.toList, tree) else tree
-
-  abstract class AppProxy(implicit @constructorOnly src: SourceFile) extends ProxyTree {
-    def app: Tree
-    override def span = app.span
-
-    def forwardTo = app
-    def canEqual(that: Any): Boolean = app.canEqual(that)
-    def productArity: Int = app.productArity
-    def productElement(n: Int): Any = app.productElement(n)
-  }
-
-  /** A wrapper indicating that its argument is an application of an extension method.
-   */
-  class ExtMethodApply(val app: Tree)(implicit @constructorOnly src: SourceFile) extends AppProxy:
-    overwriteType(app.tpe)
-      // ExtMethodApply always has wildcard type in order not to prompt any further adaptations
-      // such as eta expansion before the method is fully applied.
 }
 
 trait Applications extends Compatibility {

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

@@ -648,6 +648,14 @@ object Checking {
           else "Cannot override non-inline parameter with an inline parameter",
           p1.srcPos)
 
+  def checkConversionsSpecific(from: Type, to: Type, pos: SrcPos)(using Context): Unit =
+    def fail(part: String, tp: Type) =
+      report.error(em"the $part of an implicit conversion must be more specific than $tp", pos)
+    if to.isRef(defn.AnyValClass, skipRefined = false)
+       || to.isRef(defn.ObjectClass, skipRefined = false)
+    then fail("result", to)
+    if from.isBottomTypeAfterErasure
+    then fail("argument", from)
 }
 
 trait Checking {

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

@@ -19,8 +19,21 @@ import ErrorReporting._
 import reporting._
 
 object Dynamic {
-  def isDynamicMethod(name: Name): Boolean =
+  private def isDynamicMethod(name: Name): Boolean =
     name == nme.applyDynamic || name == nme.selectDynamic || name == nme.updateDynamic || name == nme.applyDynamicNamed
+
+  /** Is `tree` a reference over `Dynamic` that should be expanded to a
+   *  dyanmic `applyDynamic`, `selectDynamic`, `updateDynamic`, or `applyDynamicNamed` call?
+   */
+  def isDynamicExpansion(tree: untpd.RefTree)(using Context): Boolean =
+    isDynamicMethod(tree.name)
+    || tree.match
+          case Select(Apply(fun: untpd.RefTree, _), nme.apply)
+          if defn.isContextFunctionClass(fun.symbol.owner) =>
+            isDynamicExpansion(fun)
+          case Select(qual, nme.apply) =>
+            isDynamicMethod(qual.symbol.name) && tree.span.isSynthetic
+          case _ => false
 }
 
 object DynamicUnapply {

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

@@ -396,12 +396,13 @@ object Implicits:
   }
 
   /** A successful search
-   *  @param tree   The typed tree that needs to be inserted
-   *  @param ref    The implicit reference that succeeded
-   *  @param level  The level where the reference was found
+   *  @param tree        The typed tree that needs to be inserted
+   *  @param ref         The implicit reference that succeeded
+   *  @param level       The level where the reference was found
+   *  @param isExtension Whether the result is an extension method application
    *  @param tstate The typer state to be committed if this alternative is chosen
    */
-  case class SearchSuccess(tree: Tree, ref: TermRef, level: Int)(val tstate: TyperState, val gstate: GadtConstraint)
+  case class SearchSuccess(tree: Tree, ref: TermRef, level: Int, isExtension: Boolean = false)(val tstate: TyperState, val gstate: GadtConstraint)
   extends SearchResult with RefAndLevel with Showable
 
   /** A failed search */
@@ -802,12 +803,11 @@ trait Implicits:
         val inferred = inferImplicit(adjust(to), from, from.span)
 
         inferred match {
-          case SearchSuccess(tree, ref, _)
-            if isOldStyleFunctionConversion(ref.underlying) && !tree.isInstanceOf[Applications.ExtMethodApply] =>
-              report.migrationWarning(
-                i"The conversion ${ref} will not be applied implicitly here in Scala 3 because only implicit methods and instances of Conversion class will continue to work as implicit views.",
-                from
-              )
+          case SearchSuccess(_, ref, _, false) if isOldStyleFunctionConversion(ref.underlying) =>
+            report.migrationWarning(
+              i"The conversion ${ref} will not be applied implicitly here in Scala 3 because only implicit methods and instances of Conversion class will continue to work as implicit views.",
+              from
+            )
           case _ =>
         }
 
@@ -829,7 +829,7 @@ trait Implicits:
    */
   def inferImplicitArg(formal: Type, span: Span)(using Context): Tree =
     inferImplicit(formal, EmptyTree, span) match
-      case SearchSuccess(arg, _, _) => arg
+      case SearchSuccess(arg, _, _, _) => arg
       case fail @ SearchFailure(failed) =>
         if fail.isAmbiguous then failed
         else
@@ -937,7 +937,6 @@ trait Implicits:
           case Select(qual, _) => apply(x, qual)
           case Apply(fn, _) => apply(x, fn)
           case TypeApply(fn, _) => apply(x, fn)
-          case tree: Applications.AppProxy => apply(x, tree.app)
           case _: This => false
           case _ => foldOver(x, tree)
       }
@@ -1021,13 +1020,23 @@ trait Implicits:
               pt, locked)
           }
           pt match
-            case selProto @ SelectionProto(selName: TermName, mbrType, _, _) if cand.isExtension =>
+            case selProto @ SelectionProto(selName: TermName, mbrType, _, _) =>
+
               def tryExtension(using Context) =
                 extMethodApply(untpd.Select(untpdGenerated, selName), argument, mbrType)
-              if cand.isConversion then
+
+              def tryConversionForSelection(using Context) =
+                val converted = tryConversion
+                if !ctx.reporter.hasErrors && !selProto.isMatchedBy(converted.tpe) then
+                  // this check is needed since adapting relative to a `SelectionProto` can succeed
+                  // even if the term is not a subtype of the `SelectionProto`
+                  err.typeMismatch(converted, selProto)
+                converted
+
+              if cand.isExtension && cand.isConversion then
                 val extensionCtx, conversionCtx = ctx.fresh.setNewTyperState()
                 val extensionResult = tryExtension(using extensionCtx)
-                val conversionResult = tryConversion(using conversionCtx)
+                val conversionResult = tryConversionForSelection(using conversionCtx)
                 if !extensionCtx.reporter.hasErrors then
                   extensionCtx.typerState.commit()
                   if !conversionCtx.reporter.hasErrors then
@@ -1036,7 +1045,8 @@ trait Implicits:
                 else
                   conversionCtx.typerState.commit()
                   conversionResult
-              else tryExtension
+              else if cand.isExtension then tryExtension
+              else tryConversionForSelection
             case _ =>
               tryConversion
         }
@@ -1055,10 +1065,7 @@ trait Implicits:
               SearchFailure(adapted.withType(new MismatchedImplicit(ref, pt, argument)))
         }
       else
-        val returned =
-          if (cand.isExtension) Applications.ExtMethodApply(adapted)
-          else adapted
-        SearchSuccess(returned, ref, cand.level)(ctx.typerState, ctx.gadt)
+        SearchSuccess(adapted, ref, cand.level, cand.isExtension)(ctx.typerState, ctx.gadt)
     }
 
   /** An implicit search; parameters as in `inferImplicit` */
@@ -1121,20 +1128,13 @@ trait Implicits:
         case alt1: SearchSuccess =>
           var diff = compareAlternatives(alt1, alt2)
           assert(diff <= 0)   // diff > 0 candidates should already have been eliminated in `rank`
-          if diff == 0 then
+          if diff == 0 && alt1.isExtension && alt2.isExtension then
             // Fall back: if both results are extension method applications,
             // compare the extension methods instead of their wrappers.
-            object extMethodApply:
-              def unapply(t: Tree): Option[Type] = t match
-                case t: Applications.ExtMethodApply => Some(methPart(stripApply(t.app)).tpe)
-                case _ => None
-            end extMethodApply
-
-            (alt1.tree, alt2.tree) match
-              case (extMethodApply(ref1: TermRef), extMethodApply(ref2: TermRef)) =>
-                diff = compare(ref1, ref2)
+            def stripExtension(alt: SearchSuccess) = methPart(stripApply(alt.tree)).tpe
+            (stripExtension(alt1), stripExtension(alt2)) match
+              case (ref1: TermRef, ref2: TermRef) => diff = compare(ref1, ref2)
               case _ =>
-
           if diff < 0 then alt2
           else if diff > 0 then alt1
           else SearchFailure(new AmbiguousImplicits(alt1, alt2, pt, argument), span)
@@ -1560,7 +1560,7 @@ final class SearchRoot extends SearchHistory:
     else
       result match {
         case failure: SearchFailure => failure
-        case success @ SearchSuccess(tree, _, _) =>
+        case success: SearchSuccess =>
           import tpd._
 
           // We might have accumulated dictionary entries for by name implicit arguments
@@ -1583,7 +1583,7 @@ final class SearchRoot extends SearchHistory:
               else prune(in.map(_._2) ++ trees, out, in ++ acc)
           }
 
-          val pruned = prune(List(tree), implicitDictionary.map(_._2).toList, Nil)
+          val pruned = prune(List(success.tree), implicitDictionary.map(_._2).toList, Nil)
           myImplicitDictionary = null
           if (pruned.isEmpty) result
           else if (pruned.exists(_._2 == EmptyTree)) NoMatchingImplicitsFailure
@@ -1640,7 +1640,7 @@ final class SearchRoot extends SearchHistory:
               case tree => tree
             })
 
-            val res = resMap(tree)
+            val res = resMap(success.tree)
 
             val blk = Block(classDef :: inst :: Nil, res).withSpan(span)
 

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

@@ -38,6 +38,11 @@ object Inferencing {
     result
   }
 
+  def canDefineFurther(tp: Type)(using Context): Boolean =
+    val prevConstraint = ctx.typerState.constraint
+    isFullyDefined(tp, force = ForceDegree.all)
+    && (ctx.typerState.constraint ne prevConstraint)
+
   /** The fully defined type, where all type variables are forced.
    *  Throws an error if type contains wildcards.
    */

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

@@ -16,7 +16,7 @@ import transform.SymUtils._
 import Contexts._
 import Names.{Name, TermName}
 import NameKinds.{InlineAccessorName, InlineBinderName, InlineScrutineeName, BodyRetainerName}
-import ProtoTypes.selectionProto
+import ProtoTypes.shallowSelectionProto
 import Annotations.Annotation
 import SymDenotations.SymDenotation
 import Inferencing.isFullyDefined
@@ -1240,7 +1240,7 @@ class Inliner(call: tpd.Tree, rhsToInline: tpd.Tree)(using Context) {
 
     override def typedSelect(tree: untpd.Select, pt: Type)(using Context): Tree = {
       assert(tree.hasType, tree)
-      val qual1 = typed(tree.qualifier, selectionProto(tree.name, pt, this))
+      val qual1 = typed(tree.qualifier, shallowSelectionProto(tree.name, pt, this))
       val resNoReduce = untpd.cpy.Select(tree)(qual1, tree.name).withType(tree.typeOpt)
       val resMaybeReduced = constToLiteral(reducer.reduceProjection(resNoReduce))
       if (resNoReduce ne resMaybeReduced)

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

@@ -229,12 +229,11 @@ object ProtoTypes {
   /** Create a selection proto-type, but only one level deep;
    *  treat constructors specially
    */
-  def selectionProto(name: Name, tp: Type, typer: Typer)(using Context): TermType =
+  def shallowSelectionProto(name: Name, tp: Type, typer: Typer)(using Context): TermType =
     if (name.isConstructorName) WildcardType
-    else tp match {
+    else tp match
       case tp: UnapplyFunProto => new UnapplySelectionProto(name)
       case tp => SelectionProto(name, IgnoredProto(tp), typer, privateOK = true)
-    }
 
   /** A prototype for expressions [] that are in some unspecified selection operation
    *