Why doesn't a constexpr have a unique value or storage?

I'm using Wt::WFormModel, the examples for which represent field names as static const char*'s. I made a bunch of changes to my project at one time, one of which was changing the field representation to be constexpr. After much troubleshooting, I found this statement in the WFormModel documentation:

The Field type is const char *, and instead of string comparisons to identify fields, pointer comparisons are used. Thus you really should use the same field constant throughout your code to refer to a given field, and you cannot use C++11 constexpr for the field constant since that does not have a unique value (since it has no storage).

How does the representation of a constexpr char* compare to the representation of a const char*?