[Bug c++/98939] New: [C++23] Implement P1787R6 "Declarations and where to find them"

["jason at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98939

            Bug ID: 98939
           Summary: [C++23] Implement P1787R6 "Declarations and where to
                    find them"
           Product: gcc
           Version: 10.0
               URL: http://wg21.link/p1787r6
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jason at gcc dot gnu.org
  Target Milestone: ---

The changes from this paper should not affect a significant amount of code;
many are clarifications that bring the wording in line with existing practice,
some are clarifications of corner cases that most code doesn't depend on, like
ambiguous lookup within a conversion-type-id.

A few changes that allow code that has been ill-formed:

conversion-type-id is added to the list of type-only contexts from P0634:

template <class T> struct A { operator T::type(); }; // OK

::template is also not required in type-only contexts:

template <class T> auto f(T t) { return static_cast<T::X<int>>(t); } // OK

Default template arguments are now complete-class contexts, like default
function arguments:

template <class T> struct A {
  template <int I = sizeof(t)> void g() { } // OK
  T t;
};

One change that might break a small amount of existing code:

Since lookup for a name after . or -> now happens first in the scope of the
object, .template is required in dependent.template X<...> even if a definition
of X would be found by unqualified lookup.

template <int> struct X { void f(); };
template <class T> void g(T t) { t.X<2>::f(); } // error, needs .template