[LLVMbugs] [Bug 11543] New: ambiguous qualified class member access lookup diagnostic unclear

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=11543

             Bug #: 11543
           Summary: ambiguous qualified class member access lookup
                    diagnostic unclear
           Product: clang
           Version: unspecified
          Platform: PC
        OS/Version: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: C++
        AssignedTo: unassignedclangbugs at nondot.org
        ReportedBy: richard-llvm at metafoo.co.uk
                CC: dgregor at apple.com, llvmbugs at cs.uiuc.edu
    Classification: Unclassified


This question came up on stackoverflow:

http://stackoverflow.com/questions/8100492/ambiguous-member-access-expression-is-clang-rejecting-valid-code

Testcase, for posterity:

template<typename T>
class TemplateClass : public T {
 public:
  void method() {}
  template<typename U>
  static void static_method(U u) { u.TemplateClass::method(); }
};

class EmptyClass {};

int main() {
  TemplateClass<TemplateClass<EmptyClass> > c;
  TemplateClass<EmptyClass>::static_method(c);
}

Clearly our diagnostic for this code is insufficient, since it's taken a month
and a +500 rep bounty for someone to figure out why the code is ill-formed. We
say:

example.cc:6:38: error: lookup of 'TemplateClass' in member access expression
is
      ambiguous
  static void static_method(U u) { u.TemplateClass::method(); }
                                     ^
example.cc:13:3: note: in instantiation of function template specialization
     
'TemplateClass<EmptyClass>::static_method<TemplateClass<TemplateClass<EmptyClass>
      > >' requested here
  TemplateClass<EmptyClass>::static_method(c);
  ^
example.cc:2:7: note: lookup in the object type
      'TemplateClass<TemplateClass<EmptyClass> >' refers here
class TemplateClass : public T {
      ^
example.cc:2:7: note: lookup from the current scope refers here

These last two notes are clearly trying to indicate the problem, but since they
don't list the template arguments they fail to point out the problem. The
problem would be more obvious if we said:

example.cc:2:7: note: lookup in the object type
      'TemplateClass<TemplateClass<EmptyClass> >' refers here [with T =
TemplateClass<EmptyClass>]
class TemplateClass : public T {
      ^
example.cc:2:7: note: lookup from the current scope refers here [with T =
EmptyClass]

It could also be useful to mention that the lookup found an injected-class-name
(but that wouldn't help in this particular case).

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