Mark basic_json converting ctor JSON_EXPLICIT

Make JSON_USE_IMPLICIT_CONVERSIONS affect the basic_json converting
constructor.

include/nlohmann/json.hpp

@@ -937,7 +937,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     template < typename BasicJsonType,
                detail::enable_if_t <
                    detail::is_basic_json<BasicJsonType>::value&& !std::is_same<basic_json, BasicJsonType>::value, int > = 0 >
-    basic_json(const BasicJsonType& val)
+    JSON_EXPLICIT basic_json(const BasicJsonType& val)
     {
         using other_boolean_t = typename BasicJsonType::boolean_t;
         using other_number_float_t = typename BasicJsonType::number_float_t;
@@ -1760,7 +1760,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
                    int > = 0 >
     BasicJsonType get_impl(detail::priority_tag<2> /*unused*/) const
     {
-        return *this;
+        return BasicJsonType(*this);
     }
 
     /*!

single_include/nlohmann/json.hpp

@@ -19303,7 +19303,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     template < typename BasicJsonType,
                detail::enable_if_t <
                    detail::is_basic_json<BasicJsonType>::value&& !std::is_same<basic_json, BasicJsonType>::value, int > = 0 >
-    basic_json(const BasicJsonType& val)
+    JSON_EXPLICIT basic_json(const BasicJsonType& val)
     {
         using other_boolean_t = typename BasicJsonType::boolean_t;
         using other_number_float_t = typename BasicJsonType::number_float_t;
@@ -20126,7 +20126,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
                    int > = 0 >
     BasicJsonType get_impl(detail::priority_tag<2> /*unused*/) const
     {
-        return *this;
+        return BasicJsonType(*this);
     }
 
     /*!

tests/src/unit-alt-string.cpp

@@ -343,7 +343,11 @@ TEST_CASE("alternative string type")
             SECTION("json to alt_json")
             {
                 json j("foo");
+#if JSON_USE_IMPLICIT_CONVERSIONS
                 alt_json aj = j;
+#else
+                alt_json aj = alt_json(j);
+#endif
 
                 alt_string as = aj.dump();
                 CHECK(j.is_string());
@@ -354,7 +358,11 @@ TEST_CASE("alternative string type")
             SECTION("alt_json to json")
             {
                 alt_json aj("foo");
+#if JSON_USE_IMPLICIT_CONVERSIONS
                 json j = aj;
+#else
+                json j = json(aj);
+#endif
 
                 alt_string as = aj.dump();
                 CHECK(aj.is_string());
@@ -368,7 +376,11 @@ TEST_CASE("alternative string type")
             SECTION("json to alt_json")
             {
                 json j{"foo"};
+#if JSON_USE_IMPLICIT_CONVERSIONS
                 alt_json aj = j;
+#else
+                alt_json aj = alt_json(j);
+#endif
 
                 alt_string as = aj.dump();
                 CHECK(j.is_array());
@@ -379,7 +391,11 @@ TEST_CASE("alternative string type")
             SECTION("alt_json to json")
             {
                 alt_json aj{"foo"};
+#if JSON_USE_IMPLICIT_CONVERSIONS
                 json j = aj;
+#else
+                json j = json(aj);
+#endif
 
                 alt_string as = aj.dump();
                 CHECK(aj.is_array());
@@ -393,7 +409,11 @@ TEST_CASE("alternative string type")
             SECTION("json to alt_json")
             {
                 json j{{"foo", {"bar", "baz"}}};
+#if JSON_USE_IMPLICIT_CONVERSIONS
                 alt_json aj = j;
+#else
+                alt_json aj = alt_json(j);
+#endif
 
                 alt_string as = aj.dump();
                 CHECK(j.is_object());
@@ -404,7 +424,11 @@ TEST_CASE("alternative string type")
             SECTION("alt_json to json")
             {
                 alt_json aj{{"foo", {"bar", "baz"}}};
+#if JSON_USE_IMPLICIT_CONVERSIONS
                 json j = aj;
+#else
+                json j = json(aj);
+#endif
 
                 alt_string as = aj.dump();
                 CHECK(aj.is_object());
@@ -418,7 +442,11 @@ TEST_CASE("alternative string type")
             SECTION("json to alt_json")
             {
                 auto j = json::binary({1, 2, 3, 4}, 128);
+#if JSON_USE_IMPLICIT_CONVERSIONS
                 alt_json aj = j;
+#else
+                alt_json aj = alt_json(j);
+#endif
 
                 alt_string as = aj.dump();
                 CHECK(j.is_binary());
@@ -429,7 +457,11 @@ TEST_CASE("alternative string type")
             SECTION("alt_json to json")
             {
                 auto aj = alt_json::binary({1, 2, 3, 4}, 128);
+#if JSON_USE_IMPLICIT_CONVERSIONS
                 json j = aj;
+#else
+                json j = json(aj);
+#endif
 
                 alt_string as = aj.dump();
                 CHECK(aj.is_binary());

tests/src/unit-regression1.cpp

@@ -1485,7 +1485,11 @@ TEST_CASE("regression tests 1")
 
     SECTION("issue #972 - Segmentation fault on G++ when trying to assign json string literal to custom json type")
     {
+#if JSON_USE_IMPLICIT_CONVERSIONS
         my_json foo = R"([1, 2, 3])"_json;
+#else
+        my_json foo = my_json(R"([1, 2, 3])"_json);
+#endif
     }
 
     SECTION("issue #977 - Assigning between different json types")
@@ -1496,7 +1500,11 @@ TEST_CASE("regression tests 1")
         CHECK(lj.size() == 1);
         CHECK(lj["x"] == 3);
         CHECK(ff.x == 3);
+#if JSON_USE_IMPLICIT_CONVERSIONS
         nlohmann::json nj = lj;                // This line works as expected
+#else
+        nlohmann::json nj = nlohmann::json(lj);                // This line works as expected
+#endif
     }
 }
 

tests/src/unit-regression2.cpp

@@ -774,12 +774,12 @@ TEST_CASE("regression tests 2")
         CHECK(test1.dump() == "{\"root\":{}}");
 
         ordered_json test2;
-        test2[ordered_json::json_pointer(p)] = json::object();
+        test2[ordered_json::json_pointer(p)] = ordered_json::object();
         CHECK(test2.dump() == "{\"root\":{}}");
 
         // json::json_pointer and ordered_json::json_pointer are the same type; behave as above
         ordered_json test3;
-        test3[json::json_pointer(p)] = json::object();
+        test3[json::json_pointer(p)] = ordered_json::object();
         CHECK(std::is_same<json::json_pointer::string_t, ordered_json::json_pointer::string_t>::value);
         CHECK(test3.dump() == "{\"root\":{}}");
     }
@@ -859,7 +859,8 @@ TEST_CASE("regression tests 2")
         CHECK(td.str == "value");
     }
 
-#ifdef JSON_HAS_CPP_20
+    // this is no longer supported when implicit conversions are disabled
+#if defined(JSON_HAS_CPP_20) && JSON_USE_IMPLICIT_CONVERSIONS
     SECTION("issue #3312 - Parse to custom class from unordered_json breaks on G++11.2.0 with C++20")
     {
         // see test for #3171

tests/src/unit-udt.cpp

@@ -713,14 +713,22 @@ TEST_CASE("different basic_json types conversions")
     SECTION("null")
     {
         json j;
+#if JSON_USE_IMPLICIT_CONVERSIONS
         custom_json cj = j;
+#else
+        custom_json cj = custom_json(j);
+#endif
         CHECK(cj == nullptr);
     }
 
     SECTION("boolean")
     {
         json j = true;
+#if JSON_USE_IMPLICIT_CONVERSIONS
         custom_json cj = j;
+#else
+        custom_json cj = custom_json(j);
+#endif
         CHECK(cj == true);
     }
 
@@ -728,49 +736,77 @@ TEST_CASE("different basic_json types conversions")
     {
         json j(json::value_t::discarded);
         custom_json cj;
+#if JSON_USE_IMPLICIT_CONVERSIONS
         CHECK_NOTHROW(cj = j);
+#else
+        CHECK_NOTHROW(cj = custom_json(j));
+#endif
         CHECK(cj.type() == custom_json::value_t::discarded);
     }
 
     SECTION("array")
     {
         json j = {1, 2, 3};
+#if JSON_USE_IMPLICIT_CONVERSIONS
         custom_json cj = j;
+#else
+        custom_json cj = custom_json(j);
+#endif
         CHECK((cj == std::vector<int> {1, 2, 3}));
     }
 
     SECTION("integer")
     {
         json j = 42;
+#if JSON_USE_IMPLICIT_CONVERSIONS
         custom_json cj = j;
+#else
+        custom_json cj = custom_json(j);
+#endif
         CHECK(cj == 42);
     }
 
     SECTION("float")
     {
         json j = 42.0;
+#if JSON_USE_IMPLICIT_CONVERSIONS
         custom_json cj = j;
+#else
+        custom_json cj = custom_json(j);
+#endif
         CHECK(cj == 42.0);
     }
 
     SECTION("unsigned")
     {
         json j = 42u;
+#if JSON_USE_IMPLICIT_CONVERSIONS
         custom_json cj = j;
+#else
+        custom_json cj = custom_json(j);
+#endif
         CHECK(cj == 42u);
     }
 
     SECTION("string")
     {
         json j = "forty-two";
+#if JSON_USE_IMPLICIT_CONVERSIONS
         custom_json cj = j;
+#else
+        custom_json cj = custom_json(j);
+#endif
         CHECK(cj == "forty-two");
     }
 
     SECTION("binary")
     {
         json j = json::binary({1, 2, 3}, 42);
+#if JSON_USE_IMPLICIT_CONVERSIONS
         custom_json cj = j;
+#else
+        custom_json cj = custom_json(j);
+#endif
         CHECK(cj.get_binary().subtype() == 42);
         std::vector<std::uint8_t> cv = cj.get_binary();
         std::vector<std::uint8_t> v = j.get_binary();
@@ -780,7 +816,11 @@ TEST_CASE("different basic_json types conversions")
     SECTION("object")
     {
         json j = {{"forty", "two"}};
+#if JSON_USE_IMPLICIT_CONVERSIONS
         custom_json cj = j;
+#else
+        custom_json cj = custom_json(j);
+#endif
         auto m = j.get<std::map<std::string, std::string>>();
         CHECK(cj == m);
     }