Remove the conversion to bool from json_view

explicit operator bool meant "refers to a value", which silently differs
from what a basic_json converts to. Use !v.is_discarded() instead.

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

docs/mkdocs/docs/examples/basic_json_view__basic_json_view.cpp

@@ -8,10 +8,10 @@ int main()
     // the default constructor is the only public one: it creates an invalid
     // (discarded) view, useful as a "no value yet" placeholder
     nlohmann::json_view v;
-    std::cout << static_cast<bool>(v) << ' ' << v.is_discarded() << '\n';
+    std::cout << v.is_discarded() << '\n';
 
     // views are trivially copyable handles (two pointers); the document owns
     // the actual data
     nlohmann::json_view copy = v;
-    std::cout << static_cast<bool>(copy) << '\n';
+    std::cout << copy.is_discarded() << '\n';
 }

docs/mkdocs/docs/examples/basic_json_view__basic_json_view.output

@@ -1,2 +1,2 @@
-false true
-false
+true
+true

docs/mkdocs/docs/examples/basic_json_view__type_predicates.cpp

@@ -35,8 +35,8 @@ int main()
     // parse without exceptions, are both discarded
     nlohmann::json_view invalid;
     json_document failed = json_document::parse("not json", /* allow_exceptions */ false);
-    std::cout << static_cast<bool>(invalid) << ' ' << invalid.is_discarded() << '\n';
-    std::cout << static_cast<bool>(failed.root()) << ' ' << failed.root().is_discarded() << '\n';
+    std::cout << invalid.is_discarded() << '\n';
+    std::cout << failed.root().is_discarded() << '\n';
 
     // type() returns the same value_t enumeration as basic_json::type()
     std::cout << (d_object.root().type() == nlohmann::json::value_t::object) << '\n';

docs/mkdocs/docs/examples/basic_json_view__type_predicates.output

@@ -7,6 +7,6 @@ true
 true true
 true false
 false
-false true
-false true
+true
+true
 true