Add test for as cast aggregate has no to_string overload

Author: filipsajdak

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

@@ -0,0 +1,5 @@
+main: () = {
+    std::variant<int, A>(42) as std::string;
+}
+
+A: type = {}

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

@@ -0,0 +1,8 @@
+In file included from mixed-as-cast-aggregate-has-no-to_string-overload.cpp:6:
+../../../include/cpp2util.h:2167:9: error: static_assert failed due to requirement 'program_violates_type_safety_guarantee<std::string, std::variant<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-as-cast-aggregate-has-no-to_string-overload.cpp2:2:11: note: in instantiation of function template specialization 'cpp2::as_<std::string, std::variant<int, A>>' requested here
+    cpp2::as_<std::string>(std::variant<int,A>(42));
+          ^
+1 error generated.

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

@@ -0,0 +1,35 @@
+
+
+//=== Cpp2 type declarations ====================================================
+
+
+#include "cpp2util.h"
+
+#line 1 "mixed-as-cast-aggregate-has-no-to_string-overload.cpp2"
+
+#line 5 "mixed-as-cast-aggregate-has-no-to_string-overload.cpp2"
+class A;
+
+
+//=== Cpp2 type definitions and function declarations ===========================
+
+#line 1 "mixed-as-cast-aggregate-has-no-to_string-overload.cpp2"
+auto main() -> int;
+
+#line 5 "mixed-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-as-cast-aggregate-has-no-to_string-overload.cpp2"
+
+
+//=== Cpp2 function definitions =================================================
+
+#line 1 "mixed-as-cast-aggregate-has-no-to_string-overload.cpp2"
+auto main() -> int{
+#line 2 "mixed-as-cast-aggregate-has-no-to_string-overload.cpp2"
+    cpp2::as_<std::string>(std::variant<int,A>(42));
+}
+

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

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