Use result of TypeTest

Author: som-snytt

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

@@ -65,7 +65,14 @@ class CheckUnused private (phaseMode: PhaseMode, suffix: String) extends MiniPha
   // import x.y; x may be rewritten x.y, also import x.z as y
   override def transformSelect(tree: Select)(using Context): tree.type =
     val name = tree.removeAttachment(OriginalName).getOrElse(nme.NO_NAME)
-    if tree.qualifier.span.isSynthetic || name.exists(_ != tree.symbol.name) then
+    if tree.span.isSynthetic && tree.symbol == defn.TypeTest_unapply then
+      tree.qualifier.tpe.underlying.finalResultType match
+      case AppliedType(_, args) => // if tycon.typeSymbol == defn.TypeTestClass
+        val res = args(1)
+        val target = res.dealias.typeSymbol
+        resolveUsage(target, target.name, res.importPrefix.skipPackageObject) // case _: T =>
+      case _ =>
+    else if tree.qualifier.span.isSynthetic || name.exists(_ != tree.symbol.name) then
       if !ignoreTree(tree) then
         resolveUsage(tree.symbol, name, tree.qualifier.tpe)
     else

tests/warn/i21525.scala

@@ -0,0 +1,20 @@
+//> using options -Wunused:imports
+
+import scala.reflect.TypeTest
+
+trait A {
+  type B
+  type C <: B
+
+  given instance: TypeTest[B, C]
+}
+
+def f(a: A, b: a.B): Boolean = {
+  import a.C
+  b match {
+    case _: C =>
+      true
+    case _ =>
+      false
+  }
+}