[llvm-bugs] [Bug 39749] New: Headers containing just #error harm __has_include

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=39749

            Bug ID: 39749
           Summary: Headers containing just #error harm __has_include
           Product: libc++
           Version: unspecified
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: All Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: bmerry at gmail.com
                CC: llvm-bugs at lists.llvm.org, mclow.lists at gmail.com

It looks like the latest version of <experimental/optional>
(https://llvm.org/svn/llvm-project/libcxx/trunk/include/experimental/optional)
consists only of a #error to tell the user that it isn't supported anymore.

While a helpful error, it will presumably(*) break any code that uses
__has_include to detect whether this header is supported (as recommended by
N4480).

The same applies to <experimental/string_view>, and I'm guessing others
although I haven't checked.

(*) I haven't yet got my own build of libc++ trunk to verify experimentally, so
if there is some compiler magic to know that __has_include should be 0 for
these special cases then I'm less worried.

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