[Bug c++/88434] New: operator< should take precedence over template argument in basic.lookup.classref

[gieseanw+gcc at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88434

            Bug ID: 88434
           Summary: operator< should take precedence over template
                    argument in basic.lookup.classref
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gieseanw+gcc at gmail dot com
  Target Milestone: ---

Semi-related: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85570
Bug discovered here: https://stackoverflow.com/q/53712642/27678


The following code does not compile in gcc 9 (or earlier) or clang 8 (or
earlier), but does in MSVC 19.00.23506.

compile with -std=c++11 Code follows:

#include <type_traits>

using namespace std;

struct A{ int rank; };

template<typename T>
bool comp_rank(const T &a, const T &b){
    return a.rank < b.rank;
}

int main()
{
    A a{42}, b{0};
    comp_rank(a, b);
}

gcc fails with 

prog.cc: In function 'bool comp_rank(const T&, const T&)':
prog.cc:9:23: error: type/value mismatch at argument 1 in template parameter
list for 'template<class> struct std::rank'
    9 |     return a.rank < b.rank;
      |                       ^~~~
prog.cc:9:23: note:   expected a type, got 'b.rank'


This appears to violate [basic.lookup.classref] which states

In a class member access expression, if the . or -> token is immediately
followed by an identifier followed by a <, the identifier must be looked up to
determine whether the < is the beginning of a template argument list or a
less-than operator. The identifier is first looked up in the class of the
object expression. If the identifier is not found, it is then looked up in the
context of the entire postfix-expression and shall name a class template.