Document that user can only provide main in TU with CONFIG_RUNNER

Closes #1851

docs/own-main.md

@@ -11,9 +11,9 @@ The easiest way to use Catch is to let it supply ```main()``` for you and handle
 
 This is achieved by writing ```#define CATCH_CONFIG_MAIN``` before the ```#include "catch.hpp"``` in *exactly one* source file.
 
-Sometimes, though, you need to write your own version of main(). You can do this by writing ```#define CATCH_CONFIG_RUNNER``` instead. Now you are free to write ```main()``` as normal and call into Catch yourself manually.
+Sometimes, though, you need to write your own version of main(). You can do this by writing ```#define CATCH_CONFIG_RUNNER``` instead. Now you are free to write ```main()``` as normal and call into Catch yourself manually. You now have a lot of flexibility - but here are three recipes to get your started:
 
-You now have a lot of flexibility - but here are three recipes to get your started:
+**Important note: you can only provide `main` in the same file you defined `CATCH_CONFIG_RUNNER`.**
 
 ## Let Catch take full control of args and config
 

include/internal/catch_tostring.h

@@ -469,20 +469,27 @@ namespace Catch {
 #endif // CATCH_CONFIG_ENABLE_VARIANT_STRINGMAKER
 
 namespace Catch {
-    struct not_this_one {}; // Tag type for detecting which begin/ end are being selected
-
-    // Import begin/ end from std here so they are considered alongside the fallback (...) overloads in this namespace
+    // Import begin/ end from std here
     using std::begin;
     using std::end;
 
-    not_this_one begin( ... );
-    not_this_one end( ... );
+    namespace detail {
+        template <typename...>
+        struct void_type {
+            using type = void;
+        };
+
+        template <typename T, typename = void>
+        struct is_range_impl : std::false_type {
+        };
+
+        template <typename T>
+        struct is_range_impl<T, typename void_type<decltype(begin(std::declval<T>()))>::type> : std::true_type {
+        };
+    } // namespace detail
 
     template <typename T>
-    struct is_range {
-        static const bool value =
-            !std::is_same<decltype(begin(std::declval<T>())), not_this_one>::value &&
-            !std::is_same<decltype(end(std::declval<T>())), not_this_one>::value;
+    struct is_range : detail::is_range_impl<T> {
     };
 
 #if defined(_MANAGED) // Managed types are never ranges