Compatibility: c++17 std::string_view::substr() is constexpr while absl::string_view::substr() is not

[hzeller]

The C++17 std::string_view has a constexpr substr() : https://en.cppreference.com/w/cpp/string/basic_string_view/substr

While the absl version of substr() can not be used in a constexpr

abseil-cpp/absl/strings/string_view.h

Line 385 in 0033c9e

string_view substr(size_type pos, size_type n = npos) const {

This makes absl::string_view slightly less convenient in initializing more complex strings at compile time and requires workarounds.

Expectation: substr() would also work as constexpr.

For implementation, probably some functional implementation of the exception-throwing has to be done.