Reorganise typedSelectWithAdapt

Prior to the next commit, I broke up the logic into internal methods, so
some can be reused, consuming then in a big Tree#orElse chain.  I also
took the opportunity to rename the method, to more easily distinguish it
from the other typedSelect.

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

@@ -729,131 +729,155 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
     val tree = cpy.Select(tree0)(qual, selName)
     val superAccess = qual.isInstanceOf[Super]
     val rawType = selectionType(tree, qual)
-    val checkedType = accessibleType(rawType, superAccess)
 
-    def finish(tree: untpd.Select, qual: Tree, checkedType: Type): Tree =
-      val select = toNotNullTermRef(assignType(tree, checkedType), pt)
-      if selName.isTypeName then checkStable(qual.tpe, qual.srcPos, "type prefix")
-      checkLegalValue(select, pt)
-      ConstFold(select)
-
-    // If regular selection is typeable, we are done
-    if checkedType.exists then
-      return finish(tree, qual, checkedType)
+    def tryType(tree: untpd.Select, qual: Tree, rawType: Type) =
+      val checkedType = accessibleType(rawType, superAccess)
+      // If regular selection is typeable, we are done
+      if checkedType.exists then
+        val select = toNotNullTermRef(assignType(tree, checkedType), pt)
+        if selName.isTypeName then checkStable(qual.tpe, qual.srcPos, "type prefix")
+        checkLegalValue(select, pt)
+        ConstFold(select)
+      else EmptyTree
 
     // Otherwise, simplify `m.apply(...)` to `m(...)`
-    if selName == nme.apply && qual.tpe.widen.isInstanceOf[MethodType] then
-      return qual
+    def trySimplifyApply() =
+      if selName == nme.apply && qual.tpe.widen.isInstanceOf[MethodType] then
+        qual
+      else EmptyTree
 
     // Otherwise, if there's a simply visible type variable in the result, try again
     // with a more defined qualifier type. There's a second trial where we try to instantiate
     // all type variables in `qual.tpe.widen`, but that is done only after we search for
     // extension methods or conversions.
-    if couldInstantiateTypeVar(qual.tpe.widen) then
-       // there's a simply visible type variable in the result; try again with a more defined qualifier type
-       // There's a second trial where we try to instantiate all type variables in `qual.tpe.widen`,
-       // but that is done only after we search for extension methods or conversions.
-      return typedSelectWithAdapt(tree, pt, qual)
+    def tryInstantiateTypeVar() =
+      if couldInstantiateTypeVar(qual.tpe.widen) then
+        // there's a simply visible type variable in the result; try again with a more defined qualifier type
+        // There's a second trial where we try to instantiate all type variables in `qual.tpe.widen`,
+        // but that is done only after we search for extension methods or conversions.
+        typedSelectWithAdapt(tree, pt, qual)
+      else EmptyTree
 
     // Otherwise, try to expand a named tuple selection
-    val namedTupleElems = qual.tpe.widenDealias.namedTupleElementTypes
-    val nameIdx = namedTupleElems.indexWhere(_._1 == selName)
-    if nameIdx >= 0 && Feature.enabled(Feature.namedTuples) then
-      return typed(
-        untpd.Apply(
-          untpd.Select(untpd.TypedSplice(qual), nme.apply),
-          untpd.Literal(Constant(nameIdx))),
-        pt)
+    def tryNamedTupleSelection() =
+      val namedTupleElems = qual.tpe.widenDealias.namedTupleElementTypes
+      val nameIdx = namedTupleElems.indexWhere(_._1 == selName)
+      if nameIdx >= 0 && Feature.enabled(Feature.namedTuples) then
+        typed(
+          untpd.Apply(
+            untpd.Select(untpd.TypedSplice(qual), nme.apply),
+            untpd.Literal(Constant(nameIdx))),
+          pt)
+      else EmptyTree
 
     // Otherwise, map combinations of A *: B *: .... EmptyTuple with nesting levels <= 22
     // to the Tuple class of the right arity and select from that one
-    if qual.tpe.isSmallGenericTuple then
-      val elems = qual.tpe.widenTermRefExpr.tupleElementTypes.getOrElse(Nil)
-      return typedSelectWithAdapt(tree, pt, qual.cast(defn.tupleType(elems)))
+    def trySmallGenericTuple(qual: Tree, withCast: Boolean) =
+      if qual.tpe.isSmallGenericTuple then
+        if withCast then
+          val elems = qual.tpe.widenTermRefExpr.tupleElementTypes.getOrElse(Nil)
+          typedSelectWithAdapt(tree, pt, qual.cast(defn.tupleType(elems)))
+        else
+          typedSelectWithAdapt(tree, pt, qual)
+      else EmptyTree
 
     // Otherwise try an extension or conversion
-    if selName.isTermName then
-      val tree1 = tryExtensionOrConversion(
-        tree, pt, IgnoredProto(pt), qual, ctx.typerState.ownedVars, this, inSelect = true)
-      if !tree1.isEmpty then
-        return tree1
+    def tryExt(tree: untpd.Select, qual: Tree) =
+      if selName.isTermName then
+        tryExtensionOrConversion(
+          tree, pt, IgnoredProto(pt), qual, ctx.typerState.ownedVars, this, inSelect = true)
+      else EmptyTree
 
     // Otherwise, try a GADT approximation if we're trying to select a member
-    // Member lookup cannot take GADTs into account b/c of cache, so we
-    // approximate types based on GADT constraints instead. For an example,
-    // see MemberHealing in gadt-approximation-interaction.scala.
-    if ctx.gadt.isNarrowing then
-      val wtp = qual.tpe.widen
-      gadts.println(i"Trying to heal member selection by GADT-approximating $wtp")
-      val gadtApprox = Inferencing.approximateGADT(wtp)
-      gadts.println(i"GADT-approximated $wtp ~~ $gadtApprox")
-      val qual1 = qual.cast(gadtApprox)
-      val tree1 = cpy.Select(tree0)(qual1, selName)
-      val checkedType1 = accessibleType(selectionType(tree1, qual1), superAccess = false)
-      if checkedType1.exists then
-        gadts.println(i"Member selection healed by GADT approximation")
-        return finish(tree1, qual1, checkedType1)
-
-      if qual1.tpe.isSmallGenericTuple then
-        gadts.println(i"Tuple member selection healed by GADT approximation")
-        return typedSelectWithAdapt(tree, pt, qual1)
-
-      val tree2 = tryExtensionOrConversion(tree1, pt, IgnoredProto(pt), qual1, ctx.typerState.ownedVars, this, inSelect = true)
-      if !tree2.isEmpty then
-        return tree2
+    def tryGadt() =
+      if ctx.gadt.isNarrowing then
+        // Member lookup cannot take GADTs into account b/c of cache, so we
+        // approximate types based on GADT constraints instead. For an example,
+        // see MemberHealing in gadt-approximation-interaction.scala.
+        val wtp = qual.tpe.widen
+        gadts.println(i"Trying to heal member selection by GADT-approximating $wtp")
+        val gadtApprox = Inferencing.approximateGADT(wtp)
+        gadts.println(i"GADT-approximated $wtp ~~ $gadtApprox")
+        val qual1 = qual.cast(gadtApprox)
+        val tree1 = cpy.Select(tree0)(qual1, selName)
+        tryType(tree1, qual1, selectionType(tree1, qual1))
+          .orElse(trySmallGenericTuple(qual1, withCast = false))
+          .orElse(tryExt(tree1, qual1))
+      else EmptyTree
 
     // Otherwise, if there are uninstantiated type variables in the qualifier type,
     // instantiate them and try again
-    if canDefineFurther(qual.tpe.widen) then
-      return typedSelectWithAdapt(tree, pt, qual)
+    def tryDefineFurther() =
+      if canDefineFurther(qual.tpe.widen) then
+        typedSelectWithAdapt(tree, pt, qual)
+      else EmptyTree
 
     def dynamicSelect(pt: Type) =
-      val tree2 = cpy.Select(tree0)(untpd.TypedSplice(qual), selName)
-      if pt.isInstanceOf[FunOrPolyProto] || pt == LhsProto then
-        assignType(tree2, TryDynamicCallType)
-      else
-        typedDynamicSelect(tree2, Nil, pt)
+        val tree2 = cpy.Select(tree0)(untpd.TypedSplice(qual), selName)
+        if pt.isInstanceOf[FunOrPolyProto] || pt == LhsProto then
+          assignType(tree2, TryDynamicCallType)
+        else
+          typedDynamicSelect(tree2, Nil, pt)
 
     // Otherwise, if the qualifier derives from class Dynamic, expand to a
     // dynamic dispatch using selectDynamic or applyDynamic
-    if qual.tpe.derivesFrom(defn.DynamicClass) && selName.isTermName && !isDynamicExpansion(tree) then
-      return dynamicSelect(pt)
+    def tryDynamic() =
+      if qual.tpe.derivesFrom(defn.DynamicClass) && selName.isTermName && !isDynamicExpansion(tree) then
+        dynamicSelect(pt)
+      else EmptyTree
 
     // Otherwise, if the qualifier derives from class Selectable,
     // and the selector name matches one of the element of the `Fields` type member,
     // and the selector is not assigned to,
     // expand to a typed dynamic dispatch using selectDynamic wrapped in a cast
-    if qual.tpe.derivesFrom(defn.SelectableClass) && !isDynamicExpansion(tree)
-        && pt != LhsProto
-    then
-      val pre = if !TypeOps.isLegalPrefix(qual.tpe) then SkolemType(qual.tpe) else qual.tpe
-      val fieldsType = pre.select(tpnme.Fields).dealias.simplified
-      val fields = fieldsType.namedTupleElementTypes
-      typr.println(i"try dyn select $qual, $selName, $fields")
-      fields.find(_._1 == selName) match
-        case Some((_, fieldType)) =>
-          val dynSelected = dynamicSelect(fieldType)
-          dynSelected match
-            case Apply(sel: Select, _) if !sel.denot.symbol.exists =>
-              // Reject corner case where selectDynamic needs annother selectDynamic to be called. E.g. as in neg/unselectable-fields.scala.
-              report.error(i"Cannot use selectDynamic here since it needs another selectDynamic to be invoked", tree.srcPos)
-            case _ =>
-          return dynSelected.ensureConforms(fieldType)
-        case _ =>
+    def trySelectable() =
+      if qual.tpe.derivesFrom(defn.SelectableClass) && !isDynamicExpansion(tree)
+          && pt != LhsProto
+      then
+        val pre = if !TypeOps.isLegalPrefix(qual.tpe) then SkolemType(qual.tpe) else qual.tpe
+        val fieldsType = pre.select(tpnme.Fields).dealias.simplified
+        val fields = fieldsType.namedTupleElementTypes
+        typr.println(i"try dyn select $qual, $selName, $fields")
+        fields.find(_._1 == selName) match
+          case Some((_, fieldType)) =>
+            val dynSelected = dynamicSelect(fieldType)
+            dynSelected match
+              case Apply(sel: Select, _) if !sel.denot.symbol.exists =>
+                // Reject corner case where selectDynamic needs annother selectDynamic to be called. E.g. as in neg/unselectable-fields.scala.
+                report.error(i"Cannot use selectDynamic here since it needs another selectDynamic to be invoked", tree.srcPos)
+              case _ =>
+            dynSelected.ensureConforms(fieldType)
+          case _ => EmptyTree
+      else EmptyTree
 
     // Otherwise, if the qualifier is a context bound companion, handle
     // by selecting a witness in typedCBSelect
-    if qual.tpe.typeSymbol == defn.CBCompanion then
-      val witnessSelection = typedCBSelect(tree0, pt, qual)
-      if !witnessSelection.isEmpty then return witnessSelection
+    def tryCBCompanion() =
+      if qual.tpe.typeSymbol == defn.CBCompanion then
+        typedCBSelect(tree0, pt, qual)
+      else EmptyTree
 
     // Otherwise, report an error
-    assignType(tree,
-      rawType match
-        case rawType: NamedType =>
-          inaccessibleErrorType(rawType, superAccess, tree.srcPos)
-        case _ =>
-          notAMemberErrorType(tree, qual, pt))
+    def reportAnError() =
+      assignType(tree,
+        rawType match
+          case rawType: NamedType =>
+            inaccessibleErrorType(rawType, superAccess, tree.srcPos)
+          case _ =>
+            notAMemberErrorType(tree, qual, pt))
+
+    tryType(tree, qual, rawType)
+      .orElse(trySimplifyApply())
+      .orElse(tryInstantiateTypeVar())
+      .orElse(tryNamedTupleSelection())
+      .orElse(trySmallGenericTuple(qual, withCast = true))
+      .orElse(tryExt(tree, qual))
+      .orElse(tryGadt())
+      .orElse(tryDefineFurther())
+      .orElse(tryDynamic())
+      .orElse(trySelectable())
+      .orElse(tryCBCompanion())
+      .orElse(reportAnError())
   end typedSelectWithAdapt
 
   /** Expand a selection A.m on a context bound companion A with type