Almost all C/C++ operator precedence tables I have consulted list the ternary conditional operator as having higher precedence than the assignment operators. There are a few tables, however, such as the one on wikipedia, and the one at operator-precedence.com, that place them on the same precedence level. Which is it, higher or same?
The answer for C++ is that ?: and = have the same precedence. Yes, almost every C++ operator precedence table out there is wrong.
In C it doesn't matter whether ?: is higher than = or not, because in C the ?: operator is not allowed to evaluate to an l-value, which is what it would have to do if precedence were to influence the behavior (given that they are already RTL associative). See the discussion under Luchian Crigore's answer for example.
Perhaps this error is so widespread because early C++ operator precedence tables may have been copied and extended from C tables. And perhaps the error has persisted because the only counterexample - expressions of the form a?b:c=d - are rarely used. Perhaps.