Cache only when the context corresponds to prefix

Caching a lookup of c in a.b.c is ok at a.b.
A lookup from a.p should not see the cached value.

Author: som-snytt

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

@@ -25,6 +25,7 @@ import dotty.tools.dotc.util.chaining.*
 
 import java.util.IdentityHashMap
 
+import scala.collection.immutable.LazyList
 import scala.collection.mutable, mutable.{ArrayBuilder, ListBuffer, Stack}
 
 import CheckUnused.*
@@ -63,8 +64,22 @@ class CheckUnused private (phaseMode: PhaseMode, suffix: String) extends MiniPha
           if depth < 10 && prefix.exists && !prefix.classSymbol.isEffectiveRoot then
             resolveUsage(prefix.classSymbol, nme.NO_NAME, NoPrefix)
             loopOverPrefixes(prefix.normalizedPrefix, depth + 1)
+        /*
         if tree.srcPos.isZeroExtentSynthetic then
-          loopOverPrefixes(tree.typeOpt.normalizedPrefix, depth = 0)
+          println(s"ZES ${tree.show}")
+          def prefixed(t: Type) =
+            if t.exists && !t.classSymbol.isEffectiveRoot then
+              println(s"CS ${t.classSymbol}")
+              Some(t, t.normalizedPrefix)
+            else
+              None
+          val prefixes = LazyList.unfold(tree.typeOpt)(prefixed).take(10).force
+          println(s"PREFIXES ${prefixes}")
+          for (prefix <- prefixes.drop(1))
+            ()
+        */
+        //if tree.srcPos.isZeroExtentSynthetic then
+        //  loopOverPrefixes(tree.typeOpt.normalizedPrefix, depth = 0)
         resolveUsage(tree.symbol, tree.name, tree.typeOpt.importPrefix.skipPackageObject)
     else if tree.hasType then
       resolveUsage(tree.tpe.classSymbol, tree.name, tree.tpe.importPrefix.skipPackageObject)
@@ -330,7 +345,7 @@ class CheckUnused private (phaseMode: PhaseMode, suffix: String) extends MiniPha
     def addCached(where: Context, result: Precedence): Unit =
       if where.moreProperties ne null then
         where.property(resolvedKey) match
-        case Some(resolved) =>
+        case Some(resolved) if where.owner.isLocalToBlock || where.owner.info =:= prefix =>
           resolved.record(sym, name, prefix, result)
         case none =>
 
@@ -361,7 +376,7 @@ class CheckUnused private (phaseMode: PhaseMode, suffix: String) extends MiniPha
           cur.property(resolvedKey) match
           case Some(resolved) =>
             // conservative, cache must be nested below the result context
-            if precedence.isNone then
+            if precedence.isNone && (cur.owner.isLocalToBlock || cur.owner.info =:= prefix) then
               cachePoint = cur // no result yet, and future result could be cached here
             resolved.hasRecord(sym, name, prefix)
           case none => NoPrecedence

tests/warn/i22971.scala

@@ -0,0 +1,45 @@
+//> using options -Wunused:imports -Vprint:typer
+//> abusing options -Yprint-trees:format
+
+package p:
+
+  trait Base
+  class Class extends Base
+
+  abstract class Entity[T: GetType]
+
+  class Thing extends Entity[Class]
+
+  trait GetType[T]
+
+  object GetType {
+    //implicit object GetTypeClass extends GetType[Class]
+    //given GetTypeClass: GetType[Class] = new GetType[Class] {}
+    //val GetTypeClass: GetType[Class] = new GetType[Class] {}
+    implicit val GetTypeClass: GetType[Class] = new GetType[Class] {}
+  }
+  object Main {
+    def main(args: Array[String]): Unit = {
+      import GetType.*
+      //import GetType.given
+      val e = GetTypeClass
+    }
+    //def test: Unit = summon[GetType[Class]]
+  }
+
+package q:
+
+  class C:
+    def f =
+      import p.*
+      GetType.GetTypeClass
+    def g =
+      import p.GetType.*
+      GetTypeClass
+      /*
+    def h =
+      import p.*
+      summon[GetType[Class]]
+
+  class D extends p.Entity[p.Class]
+  */