Construct explicitly in get_to() and to_json(std::optional)

With JSON_USE_IMPLICIT_CONVERSIONS=0 the conversion from a basic_json
with a different string type is now explicit, but two library paths
still assigned such a value implicitly and failed to compile inside the
library:

- get_to() with a basic_json target (the #2175 overload) did
  `v = *this`, so json(42).get_to(alt_json&) broke although
  get<alt_json>() works.
- to_json(BasicJsonType&, const std::optional<T>&) is constrained on
  std::is_constructible (which accepts the explicit constructor) but did
  `j = *opt`, so converting a std::optional<alt_json> into a json broke.

Both now construct the value explicitly, as get_impl() already does.

Also replace static_cast<bool>(JSON_USE_IMPLICIT_CONVERSIONS) with a
comparison: clang-tidy's modernize-use-bool-literals rejected the cast
of the integer literal the macro expands to, failing ci_clang_tidy.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

include/nlohmann/detail/conversions/to_json.hpp

@@ -291,7 +291,9 @@ void to_json(BasicJsonType& j, const std::optional<T>& opt) noexcept
 {
     if (opt.has_value())
     {
-        j = *opt;
+        // explicit construction, as the conversion from a basic_json with a different
+        // string type is explicit if JSON_USE_IMPLICIT_CONVERSIONS is 0 (#2649)
+        j = BasicJsonType(*opt);
     }
     else
     {

include/nlohmann/json.hpp

@@ -1611,7 +1611,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     /// the same string type (see https://github.com/nlohmann/json/issues/2649)
     template<typename BasicJsonType>
     using is_implicitly_convertible_basic_json = std::integral_constant < bool,
-          static_cast<bool>(JSON_USE_IMPLICIT_CONVERSIONS)
+          (JSON_USE_IMPLICIT_CONVERSIONS != 0)
           || std::is_same<typename BasicJsonType::string_t, string_t>::value >;
 
     /// tag to select the constructor that performs the conversion from another basic_json specialization
@@ -2602,7 +2602,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
                  int> = 0>
     ValueType & get_to(ValueType& v) const
     {
-        v = *this;
+        v = ValueType(*this);
         return v;
     }
 

single_include/nlohmann/json.hpp

@@ -6820,7 +6820,9 @@ void to_json(BasicJsonType& j, const std::optional<T>& opt) noexcept
 {
     if (opt.has_value())
     {
-        j = *opt;
+        // explicit construction, as the conversion from a basic_json with a different
+        // string type is explicit if JSON_USE_IMPLICIT_CONVERSIONS is 0 (#2649)
+        j = BasicJsonType(*opt);
     }
     else
     {
@@ -26492,7 +26494,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     /// the same string type (see https://github.com/nlohmann/json/issues/2649)
     template<typename BasicJsonType>
     using is_implicitly_convertible_basic_json = std::integral_constant < bool,
-        static_cast<bool>(JSON_USE_IMPLICIT_CONVERSIONS)
+        (JSON_USE_IMPLICIT_CONVERSIONS != 0)
         || std::is_same<typename BasicJsonType::string_t, string_t>::value >;
 
     /// tag to select the constructor that performs the conversion from another basic_json specialization
@@ -27483,7 +27485,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
                  int> = 0>
     ValueType & get_to(ValueType& v) const
     {
-        v = *this;
+        v = ValueType(*this);
         return v;
     }
 

tests/src/unit-alt-string.cpp

@@ -403,5 +403,10 @@ TEST_CASE("alternative string type")
         // (only a number is converted here, as objects and strings are affected by #3425)
         CHECK(nlohmann::json(42).get<alt_json>() == 42);
         CHECK(alt_json(nlohmann::json(42)) == 42);
+
+        // get_to() also works in either case
+        alt_json a;
+        nlohmann::json(42).get_to(a);
+        CHECK(a == 42);
     }
 }