[cfe-commits] [PATCH] PR10053 Improved two-stage lookup diagnostic

[Richard Smith]

Hi again,

On Sun, June 5, 2011 04:16, Douglas Gregor wrote:
> On Jun 4, 2011, at 1:37 PM, Richard Smith wrote:
>> The attached patch improves our error recovery when handling code which
>> some versions of gcc incorrectly accept, due to an incomplete
>> implementation of two-stage name lookup.
>>
>> This example from the compatibility section on the www:
>>
>>
>> template <typename T> T Squared(T x) { return Multiply(x, x); }
>>
>>
>> int Multiply(int x, int y) { return x * y; }
>>
>>
>> int main() { Squared(5);
>> }
>>
>>
>> Now produces the following diagnostics:
>>
>>
>> my_file.cpp:2:10: error: use of undeclared identifier 'Multiply'
>> return Multiply(x, x); ^
>> my_file.cpp:10:3: note: in instantiation of function template
>> specialization 'Squared<int>' requested here Squared(5);
>> ^
>> my_file.cpp:5:5: note: viable function not candidate: declared after
>> template was defined and not in an associated namespace int Multiply(int
>> x, int y) { ^
>
> The code looks good, but I think the diagnostic wording could be improved
> considerably. The error message should say what the problem is, and the
> note should suggest the solution. For example:
>
> error: call to function 'Multiply' that is neither visible in the
> template definition nor found by argument dependent lookup
>
> note: 'Multiply' should be declared prior to the call site or in the
> namespace of one of its arguments

The attached, revised patch produces this in the case where there are no
associated namespaces:

  my_file.cpp:2:10: error: call to function 'Multiply' that is neither
visible in the template definition nor found by argument dependent
lookup
    return Multiply(x, x);
           ^
  my_file.cpp:10:3: note: in instantiation of function template
specialization 'Squared<int>' requested here
    Squared(5);
    ^
  my_file.cpp:5:5: note: 'Multiply' should be declared prior to the call site
  int Multiply(int x, int y) {
      ^

In the case where there is a single associated namespace (excluding those
within namespace std), we produce this:

  my_file2.cpp:5:13: error: call to function 'operator<<' that is neither
visible in the template definition nor found by argument dependent
lookup
    std::cout << value << "\n";
              ^
  my_file2.cpp:17:3: note: in instantiation of function template
specialization 'Dump<ns::Data>' requested here
    Dump(ns::Data());
    ^
  my_file2.cpp:12:15: note: 'operator<<' should be declared prior to the
call site or in namespace 'ns'
  std::ostream& operator<<(std::ostream& out, ns::Data data) {
                ^

Finally, if there are multiple associated namespaces for the call, we
produce a slightly tweaked version of the diagnostic you suggested:

  my_file3.cpp:5:10: error: call to function 'operator!' that is neither
visible in the template definition nor found by argument dependent
lookup
    return !value;
           ^
  my_file3.cpp:18:3: note: in instantiation of function template
specialization 'Negate<ns2::S<ns::Data> >' requested here
    Negate(ns2::S<ns::Data>());
    ^
  my_file3.cpp:15:18: note: 'operator!' should be declared prior to the
call site or in an associated namespace of one of its arguments
  ns2::S<ns::Data> operator!(ns2::S<ns::Data>);
                   ^

>> This patch only diagnoses normal unqualified names; diagnostics for
>> names with scope specifiers and for overloaded operators are not
>> similarly improved.

I've extended the patch to also cover overloaded operators. We don't fully
recover in that case, but we do produce the improved diagnostic.

Comments?

Thanks,
Richard