QJson accepts invalid escape sequences in strings when parsing JSON files

QJson accepts invalid backslash escapes in strings when parsing Json.

The attached qmake project demonstrates the problem: It passes a string containing a "\." and QJsonDocument::fromJson(...) accepts that, returning a string containing a fixed string without the "stray" backslash instead.

ECMA says that a conforming json processor should not do this. The Python Json parser correctly rejects files like these.