Dealias when looking into imports (#22889)

Fixes #22787

Author: sjrd

Diff for: compiler/src/dotty/tools/dotc/transform/CheckUnused.scala

@@ -292,7 +292,9 @@ class CheckUnused private (phaseMode: PhaseMode, suffix: String) extends MiniPha
 
     def matchingSelector(info: ImportInfo): ImportSelector | Null =
       val qtpe = info.site
-      def hasAltMember(nm: Name) = qtpe.member(nm).hasAltWith(_.symbol == sym)
+      def hasAltMember(nm: Name) = qtpe.member(nm).hasAltWith: alt =>
+           alt.symbol == sym
+        || nm.isTypeName && alt.symbol.isAliasType && alt.info.dealias.typeSymbol == sym
       def loop(sels: List[ImportSelector]): ImportSelector | Null = sels match
         case sel :: sels =>
           val matches =

Diff for: tests/warn/i22787.scala

@@ -0,0 +1,19 @@
+//> using options -Wunused:imports -Werror
+
+object tpd:
+  type Block = Trees.Block
+
+object Trees:
+  abstract case class Block(x: Int)
+  private object Block
+  val block = new Block(42) {}
+
+def f(x: Any) =
+  import tpd.Block
+  //import Trees.Block
+  x match
+  case Block(_) => "yes"
+  case _ => "no"
+
+@main def test = println:
+  f(Trees.block)