[llvm-bugs] [Bug 39072] New: valueless_by_exception() should unconditionally return false if all the constructors are noexcept

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=39072

            Bug ID: 39072
           Summary: valueless_by_exception() should unconditionally return
                    false if all the constructors are noexcept
           Product: libc++
           Version: 7.0
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: All Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: antoshkka at gmail.com
                CC: llvm-bugs at lists.llvm.org, mclow.lists at gmail.com

Right now the variant::valueless_by_exception() is defined like that:

  constexpr bool valueless_by_exception() const noexcept {
    return index() == variant_npos;
  }


Above code is quite complex for the optimizer. It may be profitable to detect
variant types that never have valueless_by_exception() state and re-implement
the function:

  constexpr bool valueless_by_exception() const noexcept {
    return !__always_valid::value /*compile-time constant*/ && index() ==
variant_npos;
  }


Such change would improve assembly for multiple functions including
comparisons, swap, get and constructors.

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