From b9850740b965b0c6ff126400f6ac52a0ec577ae1 Mon Sep 17 00:00:00 2001 From: Niels Lohmann Date: Sun, 4 Oct 2026 11:48:47 +0200 Subject: [PATCH] Add regression test for converting json to std::variant (#5595) * Add regression test for converting json to std::variant (#5066) With 3.10.5, get>() was well-formed through the string from_json overload, so the implicit conversion operator was a candidate when converting json to std::variant, 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 * 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 from libstdc++ 10's ("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 --------- Signed-off-by: Niels Lohmann --- tests/src/unit-regression2.cpp | 20 ++++++++++++++++++++ 1 file changed, 20 insertions(+) diff --git a/tests/src/unit-regression2.cpp b/tests/src/unit-regression2.cpp index 1ea5ac595..79ca5a770 100644 --- a/tests/src/unit-regression2.cpp +++ b/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 via the conversion operator") + { + // std::variant 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>::value, + "std::variant must not be retrievable via get<>()"); + + // clang before 7 cannot instantiate libstdc++'s std::variant +#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> 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{};