Drill into QuotePattern bindings symbol info (#22987)

Fixes #22981

The phase examines trees, not every type. Maybe this is just a matter of
adding a case to `Other` trees. This fix also works at `transformBind`.

Alternatively, it reflects the strategy of the phase in 3.6, that if the
mini-phase API doesn't offer structural support, then it might as well
do a custom traversal. Then the question is whether mini-phase is just
overhead of dispatch.

Besides checking imports, types could appear only in (possibly inferred)
types.

Author: sjrd

compiler/src/dotty/tools/dotc/ast/Trees.scala

@@ -741,11 +741,11 @@ object Trees {
   }
 
   /** A tree representing a quote pattern `'{ type binding1; ...; body }` or `'[ type binding1; ...; body ]`.
-   *  `QuotePattern`s are created the type checker when typing an `untpd.Quote` in a pattern context.
+   *  `QuotePattern`s are created by the type checker when typing an `untpd.Quote` in a pattern context.
    *
    *  `QuotePattern`s are checked are encoded into `unapply`s  in the `staging` phase.
    *
-   *   The `bindings` contain the list of quote pattern type variable definitions (`Bind`s) in the oreder in
+   *   The `bindings` contain the list of quote pattern type variable definitions (`Bind`s) in the order in
    *   which they are defined in the source.
    *
    *   @param  bindings  Type variable definitions (`Bind` tree)

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

@@ -154,6 +154,19 @@ class CheckUnused private (phaseMode: PhaseMode, suffix: String) extends MiniPha
   override def prepareForBind(tree: Bind)(using Context): Context =
     refInfos.register(tree)
     ctx
+  /* cf QuotePattern
+  override def transformBind(tree: Bind)(using Context): tree.type =
+    tree.symbol.info match
+    case TypeBounds(lo, hi) =>
+      def resolve(tpe: Type): Unit =
+        val sym = tpe.typeSymbol
+        if sym.exists then
+          resolveUsage(sym, sym.name, NoPrefix)
+      resolve(lo)
+      resolve(hi)
+    case _ =>
+    tree
+  */
 
   override def prepareForValDef(tree: ValDef)(using Context): Context =
     if !tree.symbol.is(Deferred) && tree.rhs.symbol != defn.Predef_undefined then
@@ -252,7 +265,17 @@ class CheckUnused private (phaseMode: PhaseMode, suffix: String) extends MiniPha
     case Splice(expr) =>
       transformAllDeep(expr)
     case QuotePattern(bindings, body, quotes) =>
-      bindings.foreach(transformAllDeep)
+      bindings.foreach:
+        case b @ Bind(_, _) =>
+          b.symbol.info match
+          case TypeBounds(lo, hi) =>
+            def resolve(tpe: Type): Unit =
+              val sym = tpe.typeSymbol
+              if sym.exists then
+                resolveUsage(sym, sym.name, NoPrefix)
+            resolve(lo)
+            resolve(hi)
+          case _ =>
       transformAllDeep(body)
       transformAllDeep(quotes)
     case SplicePattern(body, typeargs, args) =>

tests/warn/i22981.scala

@@ -0,0 +1,9 @@
+//> using options -Werror -Wunused:all
+import scala.quoted.*
+
+object test {
+  import scala.concurrent.duration.FiniteDuration
+
+  def applyImpl[A: Type](using Quotes): Expr[Unit] =
+    Type.of[A] match { case '[type d <: FiniteDuration; d] => '{ () } }
+}