Give operator>> a strong exception-safety guarantee (#5695)

operator>> parsed directly into its basic_json& target, so a parse
error left the target holding whatever was parsed before the error
instead of its previous value. With JSON_DIAGNOSTICS=1, that partial
value also violated the class invariant, because the parent pointers
of an array or object's elements are only set when the container is
closed, which a failed parse never reaches; copying such a value then
aborted in assert_invariant().

Fix it the way basic_json::parse() already handles this: parse into a
temporary and move it into the target only once parsing succeeds, so
the target is left unchanged if an exception is thrown.

Fixes #5652.

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

docs/mkdocs/docs/api/operator_gtgt.md

@@ -18,6 +18,10 @@ Deserializes an input stream to a JSON value.
 
 the stream `i`
 
+## Exception safety
+
+Strong guarantee: if an exception is thrown, there are no changes in `j`.
+
 ## Exceptions
 
 - Throws [`parse_error.101`](../home/exceptions.md#jsonexceptionparse_error101) in case of an unexpected token, or if
@@ -125,3 +129,5 @@ being read.
   the stream; planned to become the default in version 4.0.0.
 - Fixed a null pointer dereference for an `std::istream` without a stream buffer (now throws `parse_error.101`), and a
   crash (`std::terminate`) when `i` has `eofbit` in its exception mask, in version 3.13.0.
+- Changed to the strong exception safety guarantee in version 3.13.0: `j` is no longer left with a partially parsed
+  value if parsing throws.