Drill through type lambda for tree symbol

Select node in `new p.C[T]` has a lambda type,
so the symbol `C` is found in its result type.
The TypeApply has a symbol which is the constructor of C.
Both trees are used to register `C` as used.

Author: som-snytt

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

@@ -82,7 +82,10 @@ class CheckUnused private (phaseMode: PhaseMode, suffix: String) extends MiniPha
       && tree.qualifier.tpe.match
         case ThisType(_) | SuperType(_, _) => false
         case qualtpe => qualtpe.isStable
-    if tree.srcPos.isSynthetic && tree.symbol == defn.TypeTest_unapply then
+    val sym = tree.symbol
+            .orElse:
+              tree.typeOpt.resultType.typeSymbol
+    if tree.srcPos.isSynthetic && sym == defn.TypeTest_unapply then
       tree.qualifier.tpe.underlying.finalResultType match
       case AppliedType(tycon, args) =>
         val res =
@@ -92,11 +95,11 @@ class CheckUnused private (phaseMode: PhaseMode, suffix: String) extends MiniPha
         val target = res.dealias.typeSymbol
         resolveUsage(target, target.name, res.importPrefix.skipPackageObject) // case _: T =>
       case _ =>
-    else if isImportable || name.exists(_ != tree.symbol.name) then
+    else if isImportable || name.exists(_ != sym.name) then
       if !ignoreTree(tree) then
-        resolveUsage(tree.symbol, name, tree.qualifier.tpe)
+        resolveUsage(sym, name, tree.qualifier.tpe)
     else if !ignoreTree(tree) then
-      refUsage(tree.symbol)
+      refUsage(sym)
     refInfos.isAssignment = false
     tree
 
@@ -126,6 +129,11 @@ class CheckUnused private (phaseMode: PhaseMode, suffix: String) extends MiniPha
     case _ =>
     tree
 
+  override def transformTypeApply(tree: TypeApply)(using Context): tree.type =
+    if tree.symbol.exists && tree.symbol.isConstructor then
+      refUsage(tree.symbol.owner) // redundant with use of resultType in transformSelect of fun
+    tree
+
   override def prepareForAssign(tree: Assign)(using Context): Context =
     tree.lhs.putAttachment(AssignmentTarget, ()) // don't take LHS reference as a read
     ctx

tests/warn/i23694.scala

@@ -0,0 +1,27 @@
+//> using options -Wunused:privates
+
+trait A:
+  def get: Any = new A.C // select has type [T] =>> C[T]
+  def cc: Any = new A.CC // select has type CC
+
+object A:
+  private class C[T]
+  private type CC[T] = C[T]
+
+trait B:
+  def get: Any = new B.C
+
+object B:
+  private class C
+
+// duplicate issue #23960
+package net.marek.tyre.automaton:
+
+  private[tyre] class TyreCompiler[IN <: Tuple, R]:
+    private def compile[IS <: Tuple, T](xs: List[T]): String = xs match // warn
+      case _: List[t] =>
+        Loop[IS, t](null.asInstanceOf[t]).build
+    private class Loop[IS <: Tuple, T](
+      val i: T,
+    ):
+      def build = "27"