Workaround for gcc 6 (#3810)

Signed-off-by: Vladislav Shchapov <vladislav@shchapov.ru>
diff --git a/doc/api.rst b/doc/api.rst
index cba434f..76e3bf6 100644
--- a/doc/api.rst
+++ b/doc/api.rst
@@ -179,7 +179,8 @@
   struct fmt::formatter<point> : nested_formatter<double> {
     auto format(point p, format_context& ctx) const {
       return write_padded(ctx, [=](auto out) {
-        return format_to(out, "({}, {})", nested(p.x), nested(p.y));
+        return format_to(out, "({}, {})", this->nested(p.x),
+                         this->nested(p.y));
       });
     }
   };
diff --git a/test/format-test.cc b/test/format-test.cc
index 0557564..f735ffd 100644
--- a/test/format-test.cc
+++ b/test/format-test.cc
@@ -1765,7 +1765,8 @@
 template <> struct formatter<point> : nested_formatter<double> {
   auto format(point p, format_context& ctx) const -> decltype(ctx.out()) {
     return write_padded(ctx, [this, p](auto out) -> decltype(out) {
-      return fmt::format_to(out, "({}, {})", nested(p.x), nested(p.y));
+      return fmt::format_to(out, "({}, {})", this->nested(p.x),
+                            this->nested(p.y));
     });
   }
 };