[Bug c++/95568] New: No CTAD with list initialization within requires-clause

[Bug c++/95568] New: No CTAD with list initialization within requires-clause

[johelegp at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95568

            Bug ID: 95568
           Summary: No CTAD with list initialization within
                    requires-clause
           Product: gcc
           Version: 10.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: johelegp at gmail dot com
  Target Milestone: ---

See https://godbolt.org/z/Ljt-85.
```C++
template<class T> struct X { T x; };
template<class T> concept Y = requires { X{0}; };
```

[Bug c++/95568] No CTAD with list initialization within requires-clause

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95568

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |10.2
   Last reconfirmed|                            |2020-06-16
             Status|UNCONFIRMED                 |NEW
                 CC|                            |ppalka at gcc dot gnu.org
     Ever confirmed|0                           |1

--- Comment #1 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Confirmed.

[Bug c++/95568] No CTAD with list initialization within requires-clause

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95568

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |mpolacek at gcc dot gnu.org

--- Comment #2 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
The problem seems to be not that we're in a require-clause but that we're in a
template, the following is also rejected:

template<class T> struct X { T x; };
template<class T> void g () { X{0}; }

[Bug c++/95568] No CTAD with list initialization within requires-clause

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95568

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jason at gcc dot gnu.org

--- Comment #3 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
So far my guess is that this doesn't work because in a template
finish_decltype_type will create a DECLTYPE_TYPE in the deduction guide, so
we'll have

template<class T> X(decltype (X<T>::x))-> X<T>

deduction guide.  type_unification_real then tries to deduce T from a P/A pair
decltype (X<T>::x) and 0.  But:

The non-deduced contexts are:
-- The expression of a decltype-specifier.

and

If a template parameter is used only in non-deduced contexts and is not
explicitly specified, template argument deduction fails.

[Bug c++/95568] No CTAD with list initialization within requires-clause

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95568

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED
           Assignee|unassigned at gcc dot gnu.org      |mpolacek at gcc dot gnu.org

--- Comment #4 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
A patch I've been playing with

--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -28767,7 +28767,11 @@ do_class_deduction (tree ptype, tree tmpl, tree init,
     }

   if (tree guide = maybe_aggr_guide (tmpl, init, args))
-    cands = lookup_add (guide, cands);
+    {
+      if (dependent_type_p (TREE_TYPE (guide)))
+   return ptype;
+      cands = lookup_add (guide, cands);
+    }

   tree call = error_mark_node;

[Bug c++/95568] No CTAD with list initialization within requires-clause

[johelegp at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95568

--- Comment #5 from Johel Ernesto Guerrero Peña <johelegp at gmail dot com> ---
(In reply to Marek Polacek from comment #2)
> The problem seems to be not that we're in a require-clause but that we're in
> a template, the following is also rejected:
> 
> template<class T> struct X { T x; };
> template<class T> void g () { X{0}; }

Then this may be related to https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95434.

[Bug c++/95568] No CTAD with list initialization within templated entity

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95568

--- Comment #6 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Marek Polacek <mpolacek@gcc.gnu.org>:

https://gcc.gnu.org/g:b1005f553d3543bb56dc6b9b34ee35455d697ca4

commit r11-1713-gb1005f553d3543bb56dc6b9b34ee35455d697ca4
Author: Marek Polacek <polacek@redhat.com>
Date:   Tue Jun 23 18:07:34 2020 -0400

    c++: Fix CTAD for aggregates in template [PR95568]

    95568 complains that CTAD for aggregates doesn't work within
    requires-clause and it turned out that it doesn't work when we try
    the deduction in a template.  The reason is that maybe_aggr_guide
    creates a guide that can look like this

      template<class T> X(decltype (X<T>::x))-> X<T>

    where the parameter is a decltype, which is a non-deduced context.  So
    the subsequent build_new_function_call fails because unify_one_argument
    can't deduce anything from it ([temp.deduct.type]: "If a template
    parameter is used only in non-deduced contexts and is not explicitly
    specified, template argument deduction fails.")

    Those decltypes come from finish_decltype_type.  We can just use
    TREE_TYPE instead.  I pondered using unlowered_expr_type, but that
    didn't make any difference for the FIELD_DECLs I saw in
    class-deduction-aggr6.C.

    gcc/cp/ChangeLog:

            PR c++/95568
            * pt.c (collect_ctor_idx_types): Use TREE_TYPE.

    gcc/testsuite/ChangeLog:

            PR c++/95568
            * g++.dg/cpp2a/class-deduction-aggr5.C: New test.
            * g++.dg/cpp2a/class-deduction-aggr6.C: New test.

[Bug c++/95568] No CTAD with list initialization within templated entity

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95568

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |FIXED
             Status|ASSIGNED                    |RESOLVED

--- Comment #7 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Fixed.