Recover the denotation of constant-folded selections

With this change, the only term selections without a symbol after Typer
come from polymorphic function calls or outer selects. This should be
good enough to let us use SELECTin in all situations where overloads can
appear as #11210 is attempting.

Author: smarter

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

@@ -4,7 +4,7 @@ package ast
 
 import core._
 import Types._, Names._, NameOps._, Flags._, util.Spans._, Contexts._, Constants._
-import typer.ProtoTypes
+import typer.{ ConstFold, ProtoTypes }
 import SymDenotations._, Symbols._, Denotations._, StdNames._, Comments._
 import language.higherKinds
 import collection.mutable.ListBuffer
@@ -408,6 +408,13 @@ object Trees {
   case class Select[-T >: Untyped] private[ast] (qualifier: Tree[T], name: Name)(implicit @constructorOnly src: SourceFile)
     extends RefTree[T] {
     type ThisTree[-T >: Untyped] = Select[T]
+
+    override def denot(using Context): Denotation = typeOpt match
+      case ConstantType(_) if ConstFold.foldedUnops.contains(name) =>
+        // Recover the denotation of a constant-folded selection
+        qualifier.typeOpt.member(name).atSignature(Signature.NotAMethod, name)
+      case _ =>
+        super.denot
   }
 
   class SelectWithSig[-T >: Untyped] private[ast] (qualifier: Tree[T], name: Name, val sig: Signature)(implicit @constructorOnly src: SourceFile)

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

@@ -380,14 +380,24 @@ class TreeChecker extends Phase with SymTransformer {
     override def typedSelect(tree: untpd.Select, pt: Type)(using Context): Tree = {
       assert(tree.isTerm || !ctx.isAfterTyper, tree.show + " at " + ctx.phase)
       val tpe = tree.typeOpt
+
+      // Polymorphic apply methods stay structural until Erasure
+      val isPolyFunctionApply = (tree.name eq nme.apply) && (tree.qualifier.typeOpt <:< defn.PolyFunctionType)
+      // Outer selects are pickled specially so don't require a symbol
+      val isOuterSelect = tree.name.is(OuterSelectName)
+      if !(tree.isType || isPolyFunctionApply || isOuterSelect) then
+        val denot = tree.denot
+        assert(denot.exists, i"Selection $tree with type $tpe does not have a denotation")
+        assert(denot.symbol.exists, i"Denotation $denot of selection $tree with type $tpe does not have a symbol")
+
       val sym = tree.symbol
       val symIsFixed = tpe match {
         case tpe: TermRef => ctx.erasedTypes || !tpe.isMemberRef
         case _ => false
       }
       if (sym.exists && !sym.is(Private) &&
           !symIsFixed &&
-          !tree.name.is(OuterSelectName)) { // outer selects have effectively fixed symbols
+          !isOuterSelect) { // outer selects have effectively fixed symbols
         val qualTpe = tree.qualifier.typeOpt
         val member =
           if (sym.is(Private)) qualTpe.member(tree.name)
@@ -403,6 +413,7 @@ class TreeChecker extends Phase with SymTransformer {
                    |qualifier type      : ${tree.qualifier.typeOpt}
                    |tree type           : ${tree.typeOpt} of class ${tree.typeOpt.getClass}""")
       }
+
       checkNotRepeated(super.typedSelect(tree, pt))
     }
 

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

@@ -23,7 +23,7 @@ object ConstFold:
     nme.LT, nme.GT, nme.LE, nme.GE, nme.LSL, nme.LSR, nme.ASR,
     nme.ADD, nme.SUB, nme.MUL, nme.DIV, nme.MOD)
 
-  private val foldedUnops = Set[Name](
+  val foldedUnops = Set[Name](
     nme.UNARY_!, nme.UNARY_~, nme.UNARY_+, nme.UNARY_-)
 
   def Apply[T <: Apply](tree: T)(using Context): T =