Wunused: fix import false positive in constructors

Author: szymon-rd

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

@@ -102,6 +102,7 @@ class CheckUnused private (phaseMode: CheckUnused.PhaseMode, suffix: String, _ke
     pushInBlockTemplatePackageDef(tree)
 
   override def prepareForTemplate(tree: tpd.Template)(using Context): Context =
+    traverser.traverse(tree.constr)
     pushInBlockTemplatePackageDef(tree)
 
   override def prepareForPackageDef(tree: tpd.PackageDef)(using Context): Context =

tests/pos/i19252.scala

@@ -0,0 +1,10 @@
+//> using options -Werror -Wunused:imports
+package foo {
+  trait D1
+}
+
+object Bug:
+  import foo.D1
+  class Cl(d1: D1):
+    import foo.D1
+end Bug