I imagine it's (implicitly?) referring to avoiding whole-of-program analysis.

For example, given a declaration

  int* func(int* a);

What's the relationship between the return value and the input? You can't know without diving into 'func' itself; they could be the same pointer or it could return a freshly allocated pointer, without getting into the even more esoteric options.

Trying to solve this without recursively analysing a whole program at once is infeasible.

Rust's approach was to require more information to be provided by function definitions, but that's new syntax, and not backwards compatible, so not a palatable option for C++.