Can you assign a substring of a std::string to itself?

No.

This operation is defined by [string::assign]/4:

basic_string& assign(const basic_string& str, size_type pos,
    size_type n = npos);

Effects: Determines the effective length rlen of the string to assign as the smaller of n and str.size() - pos and calls assign(str.data() + pos rlen).

(dat typo)

Then:

basic_string& assign(const charT* s, size_type n);

Effects: Replaces the string controlled by *this with a string of length n whose elements are a copy of those pointed to by s.

Nothing about this says anything about whether str.assign(str, 0) is at all safe (in particular, we have no way of knowing when the copy of each character will occur!).

Therefore I strongly suggest you avoid doing it.