Make cross-string-type basic_json conversion explicit without implicit conversions (#5591)

* Make cross-string-type basic_json conversion explicit without implicit conversions

The converting constructor from another basic_json specialization was
always implicit, so a value with a different string_t (std::wstring, a
string with a custom allocator, ...) silently converted into a temporary,
e.g. when passed to a function taking const nlohmann::json&. Such
conversions do not produce correct values (#3425), and
JSON_USE_IMPLICIT_CONVERSIONS=0 did not catch them.

When JSON_USE_IMPLICIT_CONVERSIONS is 0, the constructor is now explicit
if the string types differ. Specializations sharing a string type (json
and ordered_json, different serializers or object maps) stay implicitly
convertible, so the NLOHMANN_DEFINE_TYPE_* macros keep working with
nested json members. get<BasicJsonType>() constructs explicitly and
works in both modes.

Fixes #2649.

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

* 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>

---------

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

docs/mkdocs/docs/api/basic_json/basic_json.md

@@ -293,6 +293,15 @@ basic_json(basic_json&& other) noexcept;
         When used without parentheses around an empty initializer list, `basic_json()` is called instead of this
         function, yielding the JSON `#!json null` value.
 
+- Overload 4:
+
+    !!! info "Implicit conversion"
+
+        The conversion is implicit unless [`JSON_USE_IMPLICIT_CONVERSIONS`](../macros/json_use_implicit_conversions.md)
+        is defined to `0` and `BasicJsonType::string_t` differs from `string_t`. In that case, the constructor is
+        `explicit`, so a JSON value with a different string type is no longer silently converted, for example when it is
+        passed to a function taking `#!cpp const json&`. Write `#!cpp json(other)` or `#!cpp other.get<json>()` instead.
+
 - Overload 7:
 
     !!! info "Preconditions"
@@ -466,7 +475,8 @@ basic_json(basic_json&& other) noexcept;
 1. Since version 1.0.0.
 2. Since version 1.0.0.
 3. Since version 2.1.0.
-4. Since version 3.2.0.
+4. Since version 3.2.0. Explicit for different string types if `JSON_USE_IMPLICIT_CONVERSIONS` is `0` since
+   version 3.13.0.
 5. Since version 1.0.0.
 6. Since version 1.0.0.
 7. Since version 1.0.0. Fixed in version 3.13.0 to also check the iterator range for binary values; before, a range