Bug 98939 (P1787R6)

(In reply to Jason Merrill from comment #0)
> 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

This already works; I fixed it via DR 2413 in GCC 10.

> ::template is also not required in type-only contexts:
> 
> template <class T> auto f(T t) { return static_cast<T::X<int>>(t); } // OK

This needs to be implemented, but it's in the spirit of P0634 which I implemented.  Strongly related to DR 1478.

It's also reminiscent of CWG 96 which is assigned to me.

> 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;
> };

This is DR 1635 / bug 57314.

Since I've dealt with deferred parsing quite often recently (in the context of deferred noexcept parsing), I might as well tackle this one too.  My hope is that the very same trick of stashing the tokens and then reparsing them at the end of the class will work here too.  This will probably need some kind of tparm -> defarg mapping.

So it looks like I'm in a position to fix at least parts of this proposal, thus mine for now.


A question worth considering is whether we only want to allow the code in C++23, or whether we really want to treat those issues as DRs, and so allow the code in previous modes.  P0634 is only enabled in C++20, so it makes the best sense to me to only allow the above in C++23.

I'm having trouble with "Default template arguments are now complete-class contexts":

class C {
  template <typename...> struct _List;

  template <unsigned long, typename, bool B = value> struct S; // #1

  template <unsigned long _Sz, typename _Uint, typename... _UInts>
  struct S<_Sz, _List<_Uint, _UInts...>>; // #2

  static constexpr bool value = false;
};

#2 wants to lookup_template_class S, but S can contain a DEFERRED_PARSE, so that's not going to work.  Perhaps we have to delay finish_template_type until the end of class somehow...

(In reply to Marek Polacek from comment #2)
> class C {
>   template <typename...> struct _List;
> 
>   template <unsigned long, typename, bool B = value> struct S; // #1
> 
>   template <unsigned long _Sz, typename _Uint, typename... _UInts>
>   struct S<_Sz, _List<_Uint, _UInts...>>; // #2
> 
>   static constexpr bool value = false;
> };
> 
> #2 wants to lookup_template_class S, but S can contain a DEFERRED_PARSE, so
> that's not going to work.  Perhaps we have to delay finish_template_type
> until the end of class somehow...

As I was saying in our meeting, I think in this testcase #2 is ill-formed because it depends on a default argument that hasn't been parsed yet.

Does that mean that code like this (from type_traits) needs to be fixed?

  class __make_unsigned_selector_base
  {
  protected:
    template<typename...> struct _List { }; 

    template<typename _Tp, typename... _Up> 
      struct _List<_Tp, _Up...> : _List<_Up...>
      { static constexpr size_t __size = sizeof(_Tp); };

    template<size_t _Sz, typename _Tp, bool = (_Sz <= _Tp::__size)>
      struct __select;

    template<size_t _Sz, typename _Uint, typename... _UInts>
      struct __select<_Sz, _List<_Uint, _UInts...>, true>
      { using __type = _Uint; };

    template<size_t _Sz, typename _Uint, typename... _UInts>
      struct __select<_Sz, _List<_Uint, _UInts...>, false>
      : __select<_Sz, _List<_UInts...>>
      { }; 
  };

when parsing the default template argument I can't know if it can be parsed right away or if I need to delay parsing (unless it's a simple literal, which in this case it isn't).

Some further interesting examples:

struct A {
  template<class T, int=42> struct B;
  template<class T> struct B<T> { };     // #1
  template<class T> struct B<T, 42> { }; // #2
};

Presumably we should continue to diagnose that #2 is a redefinition of #1.


And for class-scope explicit specializations (which GCC doesn't yet support):

struct A {
  template<class T, int=T::value> void f();  // #1
  template<class T> void f();                // #2
  template<> void f<int>();                  // #3, specialization of #2
};

The above testcase was valid before P1787, is it still so?

Note I think this paper applies to C++20 too or at least part of it.

From CWG1291:
[Accepted at the November, 2020 meeting as part of paper P1787R6 and moved to DR at the February, 2021 meeting.]

(In reply to Andrew Pinski from comment #6)
> Note I think this paper applies to C++20 too or at least part of it.
> 
> From CWG1291:
> [Accepted at the November, 2020 meeting as part of paper P1787R6 and moved
> to DR at the February, 2021 meeting.]

Yes, but I think we need to be conservative about changes that break existing code.  My expectation in comment #0 that the amount of code broken would be small seems to have been over-optimistic.

Additional comments about this proposal:
https://gcc.gnu.org/pipermail/gcc-patches/2021-August/578297.html

Trying to make sense of the new C++23 deprecation in [depr.template.template], I came up with the following, based on Example 3 in [temp.names]p6:

template <typename T> struct A {
   void f(int);
   template <typename U> void f(U);
};

template <typename T>
struct B {
   template <typename T2> struct C { };
};

// deprecated: T::C is assumed to name a class template:
template <typename T, template <typename X> class TT = T::template C> 
struct D { };
D<B<int> > db;

// recommended: T::C is assumed to name a class template:
template <typename T, template <typename X> class TT = T::C> 
struct E { };
E<B<int> > db;


Currently, the recommended form does not compile, which I presume is a change to become valid with P1787.  Likewise, the deprecated form does not warn — but that is not surprising if the preferred form is not compiling yet!

Hope the example is useful.