Add regression test for converting json to std::variant<json> (#5595)

* Add regression test for converting json to std::variant<json> (#5066)

With 3.10.5, get<std::variant<json>>() was well-formed through the string
from_json overload, so the implicit conversion operator was a candidate
when converting json to std::variant<json>, and MSVC picked it over the
variant's converting constructor. The tightened constraints from #3427 and
#3604 (3.11.0) removed that path; this test guards against regressions.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Fix clang-tidy and clang 6 in the #5066 regression test

ci_clang_tidy asked for emplace_back instead of push_back. The
push_back is the point of the test: #5066 is about the implicit
conversion from json to the vector's value type, which emplace_back
would bypass. Silence the check on that line.

clang 5 and 6 cannot instantiate std::variant<json> from libstdc++ 10's
<variant> ("cannot cast private base class"), which broke
ci_test_compilers_clang (6). Tested with the CI images: clang 7 to 11
compile and pass, including clang 11 with libstdc++ 10. Skip the runtime
check for clang before 7; the static_assert still runs.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

---------

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

tests/src/unit-regression2.cpp

@@ -808,6 +808,26 @@ TEST_CASE("regression tests 2")
         CHECK(j == k);
     }
 
+#ifdef JSON_HAS_CPP_17
+    SECTION("issue #5066 - MSVC converts json to std::variant<json> via the conversion operator")
+    {
+        // std::variant<json> must not be retrievable via get<>(), because otherwise the
+        // implicit conversion operator becomes a candidate that MSVC picks over the variant's
+        // converting constructor, routing a number through the string from_json overload
+        static_assert(!nlohmann::detail::is_detected<nlohmann::detail::get_template_function, const json&, std::variant<json>>::value,
+                      "std::variant<json> must not be retrievable via get<>()");
+
+        // clang before 7 cannot instantiate libstdc++'s std::variant<json>
+#if !(defined(__clang__) && __clang_major__ < 7)
+        // push_back, not emplace_back: #5066 needs the implicit conversion
+        // from json to the vector's value type
+        std::vector<std::variant<json>> v;
+        v.push_back(json(1)); // NOLINT(hicpp-use-emplace,modernize-use-emplace)
+        CHECK(std::get<0>(v[0]) == 1);
+#endif
+    }
+#endif
+
     SECTION("issue #3669 - invalid use of incomplete type with optional member and to_json")
     {
         const Issue3669Holder h{};