[libc++] Update the CI to Clang-20 and drop Clang-17 support (#117429)

[libc++][NFC] Add missing license headers

Also standardize the license comment in several files where it was
different from what we normally do.

[libc++] restrict the expected conversion constructor not compete against copy constructor (#96101)

fixes #92676

So right now clang does not like
```
std::expected<std::any, int> e1;
auto e2

[libc++] restrict the expected conversion constructor not compete against copy constructor (#96101)

fixes #92676

So right now clang does not like
```
std::expected<std::any, int> e1;
auto e2 = e1;

```

So basically when clang tries to do overload resolution of `auto e2 =
e1;`

It finds

```
expected(const expected&); // 1. This is OK
expected(const expected<_Up, _OtherErr>&) requires __can_convert; // 2. This needs to check its constraints
```

Then in `__can_convert`, one of the check is

```
_Not<is_constructible<_Tp, expected<_Up, _OtherErr>&>>
```
which is checking
```
is_constructible<std::any, expected<_Up, _OtherErr>&>
```

Then it looks at `std::any`'s constructor
```
template < class _ValueType,
class _Tp = decay_t<_ValueType>,
class = enable_if_t< !is_same<_Tp, any>::value && !__is_inplace_type<_ValueType>::value &&
is_copy_constructible<_Tp>::value> >
any(_ValueType&& __value);
```
In the above, `is_copy_constructible<_Tp>` expands to
```
is_copy_constructible<std::expected<std::any, int>>
```

And the above goes back to the original thing we asked : copy the
`std::expected`, which goes to the overload resolution again.

```
expected(const expected&);
expected(const expected<_Up, _OtherErr>&) requires __can_convert;
```

So the second overload results in a logical cycle.

I am not a language lawyer. We could argue that clang should give up on
the second overload which has logical cycle, as the first overload is a
perfect match.

Anyway, the fix in this patch tries to short-circuiting the second
overload's constraint check: that is, if the argument matches exact same
`expected<T, E>`, we give up immediately and let the copy constructor to
deal with it

show more ...

[libc++] Fix UB in <expected> related to "has value" flag (#68552) (#68733)

The calls to std::construct_at might overwrite the previously set
__has_value_ flag in the case where the flag is overlap

[libc++] Fix UB in <expected> related to "has value" flag (#68552) (#68733)

The calls to std::construct_at might overwrite the previously set
__has_value_ flag in the case where the flag is overlapping with
the actual value or error being stored (since we use [[no_unique_address]]).
To fix this issue, this patch ensures that we initialize the
__has_value_ flag after we call std::construct_at.

Fixes #68552

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[NFC][libc++] Removes concepts tests.

Concepts should be supported by all supported compilers. This is
probably a left over from an older version of the expected patch.

Reviewed By: #libc, philnik

[NFC][libc++] Removes concepts tests.

Concepts should be supported by all supported compilers. This is
probably a left over from an older version of the expected patch.

Reviewed By: #libc, philnik

Differential Revision: https://reviews.llvm.org/D140632

show more ...

[libc++] Implement `std::expected` P0323R12

Implement `std::expected` https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p0323r12.html

Added tests

Differential Revision: https://reviews.llvm

[libc++] Implement `std::expected` P0323R12

Implement `std::expected` https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p0323r12.html

Added tests

Differential Revision: https://reviews.llvm.org/D124516

address comment

show more ...