support construction from other basic_json types

Before this patch, `basic_json` types with different template arguments
were treated as `CompatibleArrayType`. Which sometimes leads to recursive
calls and stack overflows.

This patch adds a constructor and a `get` overload to deal with
different `basic_json` types.

test/src/unit-inspection.cpp

@@ -316,8 +316,8 @@ TEST_CASE("object inspection")
     SECTION("round trips")
     {
         for (const auto& s :
-                {"3.141592653589793", "1000000000000000010E5"
-                })
+    {"3.141592653589793", "1000000000000000010E5"
+    })
         {
             json j1 = json::parse(s);
             std::string s1 = j1.dump();

test/src/unit-udt.cpp

@@ -693,6 +693,83 @@ TEST_CASE("custom serializer that does adl by default", "[udt]")
     CHECK(me == cj.get<udt::person>());
 }
 
+TEST_CASE("different basic_json types conversions")
+{
+    using json = nlohmann::json;
+
+    SECTION("null")
+    {
+        json j;
+        custom_json cj = j;
+        CHECK(cj == nullptr);
+    }
+
+    SECTION("boolean")
+    {
+        json j = true;
+        custom_json cj = j;
+        CHECK(cj == true);
+    }
+
+    SECTION("discarded")
+    {
+        json j(json::value_t::discarded);
+        custom_json cj;
+        CHECK_NOTHROW(cj = j);
+        CHECK(cj.type() == custom_json::value_t::discarded);
+    }
+
+    SECTION("array")
+    {
+        json j = {1, 2, 3};
+        custom_json cj = j;
+        CHECK((cj == std::vector<int> {1, 2, 3}));
+    }
+
+    SECTION("integer")
+    {
+        json j = 42;
+        custom_json cj = j;
+        CHECK(cj == 42);
+    }
+
+    SECTION("float")
+    {
+        json j = 42.0;
+        custom_json cj = j;
+        CHECK(cj == 42.0);
+    }
+
+    SECTION("unsigned")
+    {
+        json j = 42u;
+        custom_json cj = j;
+        CHECK(cj == 42u);
+    }
+
+    SECTION("string")
+    {
+        json j = "forty-two";
+        custom_json cj = j;
+        CHECK(cj == "forty-two");
+    }
+
+    SECTION("object")
+    {
+        json j = {{"forty", "two"}};
+        custom_json cj = j;
+        auto m = j.get<std::map<std::string, std::string>>();
+        CHECK(cj == m);
+    }
+
+    SECTION("get<custom_json>")
+    {
+        json j = 42;
+        custom_json cj = j.get<custom_json>();
+        CHECK(cj == 42);
+    }
+}
+
 namespace
 {
 struct incomplete;