[libc] Add deduction guide and LIBC_INLINE to `cpp::expected` and `cpp::unexpected`

[SchrodingerZhu]

cpp::expected inside libc's support library needs to be updated:

• functions inside a header library should be marked with LIBC_INLINE
• proper deduction guide should be added so that we can avoid extra type parameters such that <Error> in cpp::unexpected<Error>(Error::Invalid).

[llvmbot]

Hi!

This issue may be a good introductory issue for people new to working on LLVM. If you would like to work on this issue, your first steps are:

1. Check that no other contributor has already been assigned to this issue. If you believe that no one is actually working on it despite an assignment, ping the person. After one week without a response, the assignee may be changed.
2. In the comments of this issue, request for it to be assigned to you, or just create a pull request after following the steps below. Mention this issue in the description of the pull request.
3. Fix the issue locally.
4. Run the test suite locally. Remember that the subdirectories under test/ create fine-grained testing targets, so you can e.g. use make check-clang-ast to only run Clang's AST tests.
5. Create a Git commit.
6. Run git clang-format HEAD~1 to format your changes.
7. Open a pull request to the upstream repository on GitHub. Detailed instructions can be found in GitHub's documentation. Mention this issue in the description of the pull request.

If you have any further questions about this issue, don't hesitate to ask via a comment in the thread below.

[llvmbot]

@llvm/issue-subscribers-good-first-issue

Author: Schrodinger ZHU Yifan (SchrodingerZhu)

`cpp::expected` inside libc's support library needs to be updated: - functions inside a header library should be marked with `LIBC_INLINE` - proper deduction guide should be added so that we can avoid extra type parameters such that `<Error>` in `cpp::unexpected<Error>(Error::Invalid)`.

[llvmbot]

@llvm/issue-subscribers-libc

Author: Schrodinger ZHU Yifan (SchrodingerZhu)

`cpp::expected` inside libc's support library needs to be updated: - functions inside a header library should be marked with `LIBC_INLINE` - proper deduction guide should be added so that we can avoid extra type parameters such that `<Error>` in `cpp::unexpected<Error>(Error::Invalid)`.

[changkhothuychung]

can this issue be assigned to me? I would like to take this issue, thanks!

[changkhothuychung]

@SchrodingerZhu

all the functions of cpp::expected and cpp::unexpected are member functions, so they are inline by default, what functions are we trying to add LIBC_INLINE here?

[SchrodingerZhu]

LIBC_INLINE may not be the same as inline. Our policy is to always add these markers. cc @nickdesaulniers