Move always_false_v to detail namespace (ros2#2232)

Since this is a common idiom, especially under this name, we should
define the `always_false_v` template within a namespace to avoid
conflict with other libraries and user code. This could either be
`rclcpp::detail` if it's intended only for internal use or just `rclcpp`
if it's intended as a public helper. In this PR, I've initially chosen
the former.

Signed-off-by: Nathan Wiebe Neufeldt <nwiebeneufeldt@clearpath.ai>

rclcpp/include/rclcpp/any_subscription_callback.hpp

@@ -34,15 +34,15 @@
 #include "rclcpp/type_adapter.hpp"
 
 
-template<class>
-inline constexpr bool always_false_v = false;
-
 namespace rclcpp
 {
 
 namespace detail
 {
 
+template<class>
+inline constexpr bool always_false_v = false;
+
 template<typename MessageT, typename AllocatorT>
 struct MessageDeleterHelper
 {
@@ -580,7 +580,7 @@ class AnySubscriptionCallback
         }
         // condition to catch unhandled callback types
         else {  // NOLINT[readability/braces]
-          static_assert(always_false_v<T>, "unhandled callback type");
+          static_assert(detail::always_false_v<T>, "unhandled callback type");
         }
       }, callback_variant_);
     TRACETOOLS_TRACEPOINT(callback_end, static_cast<const void *>(this));
@@ -660,7 +660,7 @@ class AnySubscriptionCallback
         }
         // condition to catch unhandled callback types
         else {  // NOLINT[readability/braces]
-          static_assert(always_false_v<T>, "unhandled callback type");
+          static_assert(detail::always_false_v<T>, "unhandled callback type");
         }
       }, callback_variant_);
     TRACETOOLS_TRACEPOINT(callback_end, static_cast<const void *>(this));
@@ -790,7 +790,7 @@ class AnySubscriptionCallback
         }
         // condition to catch unhandled callback types
         else {  // NOLINT[readability/braces]
-          static_assert(always_false_v<T>, "unhandled callback type");
+          static_assert(detail::always_false_v<T>, "unhandled callback type");
         }
       }, callback_variant_);
     TRACETOOLS_TRACEPOINT(callback_end, static_cast<const void *>(this));
@@ -924,7 +924,7 @@ class AnySubscriptionCallback
         }
         // condition to catch unhandled callback types
         else {  // NOLINT[readability/braces]
-          static_assert(always_false_v<T>, "unhandled callback type");
+          static_assert(detail::always_false_v<T>, "unhandled callback type");
         }
       }, callback_variant_);
     TRACETOOLS_TRACEPOINT(callback_end, static_cast<const void *>(this));