[LLVMbugs] [Bug 20092] New: Unqualified calls of std::get lead to ADL issues

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=20092

            Bug ID: 20092
           Summary: Unqualified calls of std::get lead to ADL issues
           Product: libc++
           Version: unspecified
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: All Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: kaballo86 at hotmail.com
                CC: llvmbugs at cs.uiuc.edu, mclow.lists at gmail.com
    Classification: Unclassified

Created attachment 12689
  --> http://llvm.org/bugs/attachment.cgi?id=12689&action=edit
proposed fix

Several calls to `std::get<I>` are unqualified within the implementation, which
leads to ADL related issues such as resolving to user-defined overloads:

    #include <tuple>
    #include <utility>

    namespace foo {
      struct bar { bool operator==(bar) const { return true; } };

      template <std::size_t I>
      void get(std::tuple<foo::bar, int>) {}
    }

    int main()
    {
      std::tuple<foo::bar, int> t1, t2;
      t1 == t2; // error: invalid operands to binary expression ('void' and
'void')
    }

or perhaps less contrivedly triggering hard errors during overload resolution:

    #include <tuple>
    #include <utility>

    namespace foo {
      struct bar { bool operator==(bar) const { return true; } };

      template <std::size_t I>
      struct hard_error { static_assert(I > 0, ""); using type = void; };

      template <std::size_t I, typename T>
      typename hard_error<I>::type get(T) {}
    }

    int main()
    {
      std::tuple<foo::bar, int> t1, t2;
      t1 == t2; // error: static_assert failed ""
                // note: in instantiation of template class
'foo::hard_error<0>'
                // note: while substituting explicitly-specified template
arguments into function template 'get' 
    }

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