Add test for typle as cast aggregate has no to_string overload

Author: filipsajdak

include/cpp2util.h

@@ -1251,7 +1251,13 @@ inline auto to_string(std::tuple<Ts...> const& t) -> std::string
     } else {
         std::string out = "(" + cpp2::to_string(std::get<0>(t));
         std::apply([&out](auto&&, auto&&... args) {
-            ((out += ", " + cpp2::to_string(args)), ...);
+            ((out += ", " + []<typename T>(T const& arg) { 
+                                if constexpr (has_to_string_overload<T> || std::is_copy_constructible_v<T>) {
+                                    return cpp2::to_string(arg);
+                                } else {
+                                    return cpp2::to_string(std::ref(arg)); // cpp2::to_string(...) requires copy constructible type
+                                }
+                            }(args)), ...);
         }, t);
         out += ")";
         return out;
@@ -1310,8 +1316,7 @@ constexpr auto has_recursive_to_string_overload(C<Ts...> const&)
 template <std::same_as<std::string> C, has_to_string_overload X>
     requires not_one_of<X, std::string, std::any> && (same_type_as<X, bool> || no_brace_initializable_to<X, C>)
 auto as( X&& x ) {
-    if constexpr (specialization_of_template<X, std::variant> 
-        && requires {
+    if constexpr ( requires {
             { has_recursive_to_string_overload(x) } -> std::same_as<std::false_type>;
         }
     ) {

regression-tests/mixed-overview-of-as-casts.cpp2

@@ -246,6 +246,12 @@ main: () = {
             a : std::any = "string" as std::string;
             print("std::any{\"string\"} as std::string", (a as std::string), "string");
         }
+        {
+            t : std::tuple<int, double, std::string> = (42, 3.14, "string");
+            print("std::tuple{42, 3.14, \"string\"} as std::string", (t as std::string), "(42, 3.140000, string)");
+            print("cpp2::to_string(std::tuple<int, A>())", cpp2::to_string(std::tuple<int, A>()), "(0, (customize me - no cpp2::to_string overload exists for this type))");
+            // std::tuple<int, A>() as std::string;
+        }
     }
 }
 

regression-tests/mixed-typle-as-cast-aggregate-has-no-to_string-overload.cpp2

@@ -0,0 +1,5 @@
+main: () = {
+    std::tuple<int, A>() as std::string;
+}
+
+A: type = {}

regression-tests/test-results/apple-clang-14/mixed-typle-as-cast-aggregate-has-no-to_string-overload.cpp.output

@@ -0,0 +1,8 @@
+In file included from mixed-typle-as-cast-aggregate-has-no-to_string-overload.cpp:6:
+../../../include/cpp2util.h:2172:9: error: static_assert failed due to requirement 'program_violates_type_safety_guarantee<std::string, std::tuple<int, A>>' "One of aggregate types has no cpp2::to_string overload - if you're sure you want this, use `cpp2::to_string() to force the conversion"
+        static_assert(
+        ^
+mixed-typle-as-cast-aggregate-has-no-to_string-overload.cpp2:2:11: note: in instantiation of function template specialization 'cpp2::as_<std::string, std::tuple<int, A>>' requested here
+    cpp2::as_<std::string>(std::tuple<int,A>());
+          ^
+1 error generated.

regression-tests/test-results/mixed-typle-as-cast-aggregate-has-no-to_string-overload.cpp

@@ -0,0 +1,35 @@
+
+
+//=== Cpp2 type declarations ====================================================
+
+
+#include "cpp2util.h"
+
+#line 1 "mixed-typle-as-cast-aggregate-has-no-to_string-overload.cpp2"
+
+#line 5 "mixed-typle-as-cast-aggregate-has-no-to_string-overload.cpp2"
+class A;
+
+
+//=== Cpp2 type definitions and function declarations ===========================
+
+#line 1 "mixed-typle-as-cast-aggregate-has-no-to_string-overload.cpp2"
+auto main() -> int;
+
+#line 5 "mixed-typle-as-cast-aggregate-has-no-to_string-overload.cpp2"
+class A {
+      public: A() = default;
+      public: A(A const&) = delete; /* No 'that' constructor, suppress copy */
+      public: auto operator=(A const&) -> void = delete;
+};
+#line 6 "mixed-typle-as-cast-aggregate-has-no-to_string-overload.cpp2"
+
+
+//=== Cpp2 function definitions =================================================
+
+#line 1 "mixed-typle-as-cast-aggregate-has-no-to_string-overload.cpp2"
+auto main() -> int{
+#line 2 "mixed-typle-as-cast-aggregate-has-no-to_string-overload.cpp2"
+    cpp2::as_<std::string>(std::tuple<int,A>());
+}
+

regression-tests/test-results/mixed-typle-as-cast-aggregate-has-no-to_string-overload.cpp2.output

@@ -0,0 +1,2 @@
+mixed-typle-as-cast-aggregate-has-no-to_string-overload.cpp2... ok (all Cpp2, passes safety checks)
+