`[[nodiscard]]` should be applied to functions where discarding the
return value is most likely a correctness issue.
-
https://libcxx.llvm.org/CodingGuidelines.html#apply-nodiscard-where-relevant

...in preparation for https://github.com/llvm/llvm-project/pull/168826
as requested in the review.

Co-authored-by: Hristo Hristov <zingam@outlook.com>

This causes various runtime failures, as reported in #168628.

This reverts both #165223 and #167779

Applied `[[nodiscard]]` where relevant to smart pointers and related
functions.

- [x] - `std::unique_ptr`
- [x] - `std::shared_ptr`
- [x] - `std::weak_ptr`

See guidelines:
-
https://libcxx.llvm.org/CodingGuidelines.html#apply-nodiscard-where-relevant
- `[[nodiscard]]` should be applied to functions where discarding the
return value is most likely a correctness issue. For example a locking
constructor in unique_lock.

---------

Co-authored-by: Hristo Hristov <zingam@outlook.com>

`numeric_limits` already has an `is_signed` member. We can use that
instead of using `std::is_signed`.

This is the promised follow-up to #167779. It simply adds a test case
provided by philnik777

Fixes #167991

Currently, there are no diagnostics issued when including a deprecated
header, since the diagnostic is issued inside a system header. This
patch fixes that by using `#warning` instead, which also simplifies the
implementation of the deprecation warnings.

https://libcxx.llvm.org/CodingGuidelines.html#apply-nodiscard-where-relevant

---------

Co-authored-by: Hristo Hristov <zingam@outlook.com>
Co-authored-by: Nikolas Klauser <nikolasklauser@berlin.de>

We want to eliminate all .compile.fail.cpp tests since they are brittle:
these tests pass regardless of the specific compilation error, which
means that e.g. a mising include will render the test null.

This is not an exhaustive pass, just a few tests I stumbled upon.