I'm unsure whether this actually needs to be an out-of-line definition. My understanding is that member function bodies aren't instantiated until needed (this is what lets list<T>::sort() require op< from T only if sort() is actually called). So you should be able to define this within the class definition, without fear of prematurely instantiating the primary template for extent_type<dynamic_extent> before the explicit specialization has been seen. But if you're especially concerned about that, I think I would recommend declaring the primary template and then declaring the explicit specialization, so you can definitely define this member function within the class definition.

Within the gsl namespace, you should never need to qualify gsl::dynamic_extent - you can simply say dynamic_extent without fear of ambiguity with std::dynamic_extent (even if the user has using namespace std;). This is because unqualified name lookup will start within gsl and find the declaration there - it won't reach into the global namespace and won't consider any using-directives/using-declarations there either. The same is true for all non-functions in gsl (e.g. types, alias templates, variable templates, etc.).

It looks like there's a mix of dynamic_extent and gsl::dynamic_extent all throughout this file. I'll file an issue and separate PR to make it more cohesive.