From c4469c9f82ffb1ed079af12015ddd0515deefcb7 Mon Sep 17 00:00:00 2001
From: Niels Lohmann <mail@nlohmann.me>
Date: Sun, 27 Sep 2026 16:49:33 +0200
Subject: [PATCH] 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>
---
 tests/src/unit-regression2.cpp | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/tests/src/unit-regression2.cpp b/tests/src/unit-regression2.cpp
index b128b7a73..bb2c377ac 100644
--- a/tests/src/unit-regression2.cpp
+++ b/tests/src/unit-regression2.cpp
@@ -765,6 +765,20 @@ 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<>()");
+
+        std::vector<std::variant<json>> v;
+        v.push_back(json(1));
+        CHECK(std::get<0>(v[0]) == 1);
+    }
+#endif
 }
 
 TEST_CASE("regression test - parser callback must not lose a duplicate key's prior value")