gsl::not_null: C.85: Make swap noexcept

[maflcko]

Currently, std::is_nothrow_swappable_v<gsl::not_null<std::shared_ptr<int>>> fails. This violates https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#c85-make-swap-noexcept

So mark not_null with the appropriate noexcept.

This can be tested by compiling the test without the include header changes and observing the failure:

tests/pointers_tests.cpp:46:23: error: static assertion failed due to requirement 'noexcept(gsl::swap(a, b))': not null unique_ptr should be noexcept-swappable
   46 |         static_assert(noexcept(gsl::swap(a, b)), "not null unique_ptr should be noexcept-swappable");
      |                       ^~~~~~~~~~~~~~~~~~~~~~~~~
tests/pointers_tests.cpp:67:23: error: static assertion failed due to requirement 'noexcept(gsl::swap(a, b))': strict not null unique_ptr should be noexcept-swappable
   67 |         static_assert(noexcept(gsl::swap(a, b)), "strict not null unique_ptr should be noexcept-swappable");
      |                       ^~~~~~~~~~~~~~~~~~~~~~~~~

Similarly, clang-tidy will complain on the previous header:

$ clang-tidy --checks='performance-noexcept-swap' -p ./build/clang-23-release/ ./include/gsl/pointers 
include/gsl/pointers:148:10: warning: swap functions should be marked noexcept [performance-noexcept-swap]
  148 |     void swap(not_null<T>& other) { std::swap(ptr_, other.ptr_); }

include/gsl/pointers:155:6: warning: swap functions should be marked noexcept [performance-noexcept-swap]
  155 | void swap(not_null<T>& a, not_null<T>& b)

[carsonRadtke]

Thanks for finding this issue! Can we make this unconditionally noexcept?

C.85: Make swap noexcept

[maflcko]

Thanks for finding this issue! Can we make this unconditionally noexcept?

C.85: Make swap noexcept

Heh, thanks for the link. I should have read the docs myself, because a plain noexecpt is also the easiest to implement.

[maflcko]

I think the CI had a hiccup, because I can't see the log. Let's try again... (edit: CI looks better)

[carsonRadtke]

Thanks!