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

@@ -330,7 +330,7 @@ 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 +361,7 @@ 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 +1,34 @@
+//> using options -Wunused:imports
+
+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]
+    implicit val GetTypeClass: GetType[Class] = new GetType[Class] {}
+  }
+  object Main {
+    def main(args: Array[String]): Unit = {
+      import GetType.*
+      val e = GetTypeClass
+    }
+  }
+
+package q:
+
+  class C:
+    def f =
+      import p.*
+      GetType.GetTypeClass
+    def g =
+      import p.GetType.*
+      GetTypeClass
+  class D extends p.Entity[p.Class]