[llvm-bugs] [Bug 39604] New: Checking for optional availability does not work like advertised in the standard

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=39604

            Bug ID: 39604
           Summary: Checking for optional availability does not work like
                    advertised in the standard
           Product: libc++
           Version: unspecified
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: All Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: ldionne at apple.com
                CC: llvm-bugs at lists.llvm.org, mclow.lists at gmail.com

The following code does not compile using libc++ trunk:

----------------------------------------------------------
#if __has_include(<optional>)
#  include <optional>
#  if __cpp_lib_optional >= 201603
#    define have_optional 1
#  endif
#elif __has_include(<experimental/optional>)
#  include <experimental/optional>
#  if __cpp_lib_experimental_optional >= 201411
#    define have_optional 1
#    define experimental_optional 1
#  endif
#endif
#ifndef have_optional
#  define have_optional 0
#endif

int main() {
#ifdef have_optional
    std::optional<int> i;
#endif

#ifdef experimental_optional
    std::experimental::optional<int> j;
#endif
}
----------------------------------------------------------

The embarrassing part is that the optional-checking code is copy-pasted from
[cpp.cond] 14.1/15.

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