[conv.lval] p3.4 sentence 2 says:

If the result is an erroneous value ([basic.indet]) and the bits in the value representation are not valid for the object's type, the behavior is undefined.

It's not obvious how you would run into this case given that generally, memory is initialized for erroneous values, and this initialization can typically done so that values are valid.

To be honest, I don't understand how you can run into this case, and it's not going to be obvious to others who follow, so we should add an example of how this can happen.