diff --git a/compiler/src/dotty/tools/dotc/transform/PostTyper.scala b/compiler/src/dotty/tools/dotc/transform/PostTyper.scala
index 8e9b95a0c572..3ed498d30dc1 100644
--- a/compiler/src/dotty/tools/dotc/transform/PostTyper.scala
+++ b/compiler/src/dotty/tools/dotc/transform/PostTyper.scala
@@ -257,6 +257,13 @@ class PostTyper extends MacroTransform with IdentityDenotTransformer { thisPhase
 
     override def transform(tree: Tree)(using Context): Tree =
       try tree match {
+        case CaseDef(pat, _, _) =>
+          val gadtCtx =
+           pat.removeAttachment(typer.Typer.InferredGadtConstraints) match
+             case Some(gadt) => ctx.fresh.setGadt(gadt)
+             case None =>
+               ctx
+          super.transform(tree)(using gadtCtx)
         case tree: Ident if !tree.isType =>
           if tree.symbol.is(Inline) && !Inliner.inInlineMethod then
             ctx.compilationUnit.needsInlining = true
@@ -438,4 +445,4 @@ class PostTyper extends MacroTransform with IdentityDenotTransformer { thisPhase
     private def normalizeErasedRhs(rhs: Tree, sym: Symbol)(using Context) =
       if (sym.isEffectivelyErased) dropInlines.transform(rhs) else rhs
   }
-}
\ No newline at end of file
+}
diff --git a/compiler/src/dotty/tools/dotc/typer/Typer.scala b/compiler/src/dotty/tools/dotc/typer/Typer.scala
index 811646b8153d..214872788942 100644
--- a/compiler/src/dotty/tools/dotc/typer/Typer.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Typer.scala
@@ -66,6 +66,9 @@ object Typer {
     if (!tree.isEmpty && !tree.isInstanceOf[untpd.TypedSplice] && ctx.typerState.isGlobalCommittable)
       assert(tree.span.exists, i"position not set for $tree # ${tree.uniqueId} of ${tree.getClass} in ${tree.source}")
 
+  /** An attachment for GADT constraints that were inferred for a pattern. */
+  val InferredGadtConstraints = new Property.StickyKey[core.GadtConstraint]
+
   /** A context property that indicates the owner of any expressions to be typed in the context
    *  if that owner is different from the context's owner. Typically, a context with a class
    *  as owner would have a local dummy as ExprOwner value.
@@ -1598,6 +1601,13 @@ class Typer extends Namer
       val pat1 = indexPattern(tree).transform(pat)
       val guard1 = typedExpr(tree.guard, defn.BooleanType)
       var body1 = ensureNoLocalRefs(typedExpr(tree.body, pt1), pt1, ctx.scope.toList)
+      if ctx.gadt.nonEmpty then
+        // Store GADT constraint to later retrieve it (in PostTyper, for now).
+        // GADT constraints are necessary to correctly check bounds of type app,
+        // see tests/pos/i12226 and issue #12226. It might be possible that this
+        // will end up taking too much memory. If it does, we should just limit
+        // how much GADT constraints we infer - it's always sound to infer less.
+        pat1.putAttachment(InferredGadtConstraints, ctx.gadt)
       if (pt1.isValueType) // insert a cast if body does not conform to expected type if we disregard gadt bounds
         body1 = body1.ensureConforms(pt1)(using originalCtx)
       assignType(cpy.CaseDef(tree)(pat1, guard1, body1), pat1, body1)
diff --git a/tests/pos/i12226.scala b/tests/pos/i12226.scala
new file mode 100644
index 000000000000..b22e8b4cd428
--- /dev/null
+++ b/tests/pos/i12226.scala
@@ -0,0 +1,16 @@
+object Test extends App {
+  sealed trait P[T]
+  case class C1[T <: String](c1: T) extends P[T]
+  case class C2[T](c2: T) extends P[T]
+
+  def test[T](p: P[T], t: T): Unit = p match {
+    case C1(_) =>
+      // T <: String
+      val t : T = ???
+      val s : String = t
+      def test = new C1[T](t)
+      println(1)
+
+    case C2(_) => println(2)
+  }
+}