<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/63346>63346</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            Incorrect include-what-you-use (IWYU) mappings file for libcxx 16
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            new issue
      </td>
    </tr>

    <tr>
      <th>Assignees</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Reporter</th>
      <td>
          percona-ysorokin
      </td>
    </tr>
</table>

<pre>
    `libcxx/include/libcxx.imp` IWYU mappings file has several incorrect entries referring non-existing `#include <>` files:
* `<fwd>`
 https://github.com/llvm/llvm-project/blob/7cbf1a2591520c2491aa35339f227775f4d3adf6/libcxx/include/libcxx.imp#L30
* `<debug_utils>`
 https://github.com/llvm/llvm-project/blob/7cbf1a2591520c2491aa35339f227775f4d3adf6/libcxx/include/libcxx.imp#L25
* `<support>`
 https://github.com/llvm/llvm-project/blob/7cbf1a2591520c2491aa35339f227775f4d3adf6/libcxx/include/libcxx.imp#L39

I suggest the following mappings for `__fwd/*`:
```diff
diff --git a/libcxx_clang16.imp b/libcxx_clang16_fixed.imp
index 575ce0f..f818891 100644
--- a/libcxx_clang16.imp
+++ b/libcxx_clang16_fixed.imp
@@ -27,7 +27,16 @@
   { include: [ "@<__filesystem/.*>", "private", "<filesystem>", "public" ] },
   { include: [ "@<__format/.*>", "private", "<format>", "public" ] },
   { include: [ "@<__functional/.*>", "private", "<functional>", "public" ] },
- { include: [ "@<__fwd/.*>", "private", "<fwd>", "public" ] },
+  { include: [ "<__fwd/array.h>", "private", "<array>", "public" ] },
+  { include: [ "<__fwd/get.h>", "private", "<tuple>", "public" ] },
+  { include: [ "<__fwd/hash.h>", "private", "<functional>", "public" ] },
+  { include: [ "<__fwd/memory_resource.h>", "private", "<memory_resource>", "public" ] },
+  { include: [ "<__fwd/pair.h>", "private", "<utility>", "public" ] },
+  { include: [ "<__fwd/span.h>", "private", "<span>", "public" ] },
+ { include: [ "<__fwd/string.h>", "private", "<string>", "public" ] },
+  { include: [ "<__fwd/string_view.h>", "private", "<string_view>", "public" ] },
+  { include: [ "<__fwd/subrange.h>", "private", "<ranges>", "public" ] },
+  { include: [ "<__fwd/tuple.h>", "private", "<tuple>", "public" ] },
   { include: [ "@<__ios/.*>", "private", "<ios>", "public" ] },
   { include: [ "@<__iterator/.*>", "private", "<iterator>", "public" ] },
   { include: [ "@<__memory/.*>", "private", "<memory>", "public" ] },
```

`__debug_utils/*` has only one file `randomize_range.h`, which is used only in 3 algorithms: `std::partial_sort`, `std::nth_element` and `std::sort`, so most probably `__debug_utils/randomize_range.h` should be mapped to `<algorithm>`.

`__support/*` are included either from `__locale` or from `__threading support`, so public headers for them should be either `<thread>` (for `__support_ibm_nanosleep.h`) or `<locale>` (for everything else).

</pre>
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