[LLVMbugs] [Bug 7209] New: Seek out invalid candidates that GCC would find when overload resolution fails

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=7209

           Summary: Seek out invalid candidates that GCC would find when
                    overload resolution fails
           Product: clang
           Version: trunk
          Platform: PC
        OS/Version: All
            Status: NEW
          Keywords: quality-of-implementation
          Severity: normal
          Priority: P
         Component: C++
        AssignedTo: unassignedclangbugs at nondot.org
        ReportedBy: dgregor at apple.com
                CC: llvmbugs at cs.uiuc.edu, dgregor at apple.com


GCC compiles the following example because it performs non-ADL name lookup for
operator<< at instantiation time (which is wrong):

  #include <iostream>

  namespace N {
    struct X { };
  }
  using namespace N;

  template<typename T>
  void log(const T& t) {
    std::clog << t;
  } 

  std::ostream &operator<<(std::ostream&, const X& x);

  template void log(const X&);

This has led to some confusion, so we would like to improve the diagnostic. For
example, if overload resolution finds nothing or is ambiguous, we could
consider adding candidates that shouldn't be visible (but other compilers
sometimes see anyway), and, if those candidates are chosen, emit a specific
diagnostic about the reason they aren't actually visible.

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