fixing issues in MSVC, adding comments in MSVC-failing examples

example/can_invoke/function_object.cpp

@@ -4,6 +4,10 @@ Distributed under the Boost Software License, Version 1.0.
 (See accompanying file LICENSE.md or copy at http ://boost.org/LICENSE_1_0.txt)
 */
 
+
+// NOTE: This does not compile in MSVC, which can't handle
+// the trailing return type on the generic lambda 'add'
+
 #include <type_traits>
 #include <callable_traits/callable_traits.hpp>
 
@@ -27,11 +31,12 @@ static_assert(ct::can_invoke(subtract, 1, 2), ""); // <-- success
 static_assert(!ct::can_invoke(subtract, 1, 2, 3), "");
 static_assert(!ct::can_invoke(subtract, bad, bad), "");
 
-
 // Generic function objects (such as `add` and `multiply` below)
 // must be SFINAE-friendly in order for failing can_invoke calls
 // to actually compile. This applies to both generic lambdas and
-// templated function objects. 
+// templated function objects. Note: MSVC does not compile this
+// source code file because of the trailing return type below (MSVC
+// is non-conforming in this respect)
 
 auto add = [](auto x, auto y) -> decltype(x + y) {
     return x + y;          // ^^^^^^^^^^^^^^^^^^ explicit return type allows SFINAE
@@ -46,7 +51,6 @@ static_assert(ct::can_invoke(add, 1, 2), ""); // <-- success
 static_assert(!ct::can_invoke(add, 1, 2, 3), "");
 static_assert(!ct::can_invoke(add, bad, bad), "");
 
-
 // 'multiply' isn't SFINAE-safe, because the return type depends
 // on an expression in the body. However, it still works if we
 // only try to call can_invoke with arguments where x * y is
@@ -66,4 +70,4 @@ static_assert(!ct::can_invoke(multiply, 1, 2, 3), "");
 // the call cannot be SFINAE'd away. Error message in Clang: 
 // "invalid operands to binary expression ('Bad' and 'Bad')"
 
-int main() {}
+int main() {}