Restore disabled check for #3070 (except on MSVC) (#3421)

Restore the previously disabled check for regression #3070 on all
compilers but MSVC.

To summarize the issue:
Given namespace fs = std::filesystem.
On MSVC attempting to construct an fs::path from json results in an
ambiguous overload resolution because fs::path can be constructed from
both a std::string as well as another fs::path.
To the best of my knowledge there is no way to fix an ambiguous overload
situation involving a type we do not control and with json implicitly
converting to both std::string and fs::path.

Re-enabling the check where it compiles and keeping it disabled for MSVC
is the best we can do.

Closes #3377 and #3382.

include/nlohmann/detail/conversions/from_json.hpp

@@ -464,8 +464,7 @@ void from_json(const BasicJsonType& j, std_fs::path& p)
 {
     if (JSON_HEDLEY_UNLIKELY(!j.is_string()))
     {
-        // Not tested because of #3377 (related #3070)
-        JSON_THROW(type_error::create(302, "type must be string, but is " + std::string(j.type_name()), j)); // LCOV_EXCL_LINE
+        JSON_THROW(type_error::create(302, "type must be string, but is " + std::string(j.type_name()), j));
     }
     p = *j.template get_ptr<const typename BasicJsonType::string_t*>();
 }

single_include/nlohmann/json.hpp

@@ -4279,8 +4279,7 @@ void from_json(const BasicJsonType& j, std_fs::path& p)
 {
     if (JSON_HEDLEY_UNLIKELY(!j.is_string()))
     {
-        // Not tested because of #3377 (related #3070)
-        JSON_THROW(type_error::create(302, "type must be string, but is " + std::string(j.type_name()), j)); // LCOV_EXCL_LINE
+        JSON_THROW(type_error::create(302, "type must be string, but is " + std::string(j.type_name()), j));
     }
     p = *j.template get_ptr<const typename BasicJsonType::string_t*>();
 }

test/src/unit-regression2.cpp

@@ -748,8 +748,10 @@ TEST_CASE("regression tests 2")
         const auto j_path = j.get<nlohmann::detail::std_fs::path>();
         CHECK(j_path == text_path);
 
-        // Disabled pending resolution of #3377
-        // CHECK_THROWS_WITH_AS(nlohmann::detail::std_fs::path(json(1)), "[json.exception.type_error.302] type must be string, but is number", json::type_error);
+#ifndef _MSC_VER
+        // works everywhere but on MSVC
+        CHECK_THROWS_WITH_AS(nlohmann::detail::std_fs::path(json(1)), "[json.exception.type_error.302] type must be string, but is number", json::type_error);
+#endif
     }
 #endif