fix #1748: desugaring with StringContext in PatDef

Author: liufengyun

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

@@ -1079,6 +1079,8 @@ object desugar {
         collect(tree)
       case Tuple(trees) =>
         trees foreach collect
+      case Thicket(trees) =>
+        trees foreach collect
       case _ =>
     }
     collect(tree)

tests/pos/i1784.scala

@@ -0,0 +1,14 @@
+object Test {
+  implicit class Foo(sc: StringContext) {
+    object q {
+      def unapply(args: Any*): Option[(Any, Any)] =
+        Some((sc.parts(0), sc.parts(1)))
+    }
+  }
+
+  def f(defn: Any): Any = {
+    val q"class $name extends $parent" =  defn
+    println(name)
+    println(parent)
+  }
+}