[Bug c++/102933] New: Can't use CTAD in template argument

[Bug c++/102933] New: Can't use CTAD in template argument

[johelegp at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102933

            Bug ID: 102933
           Summary: Can't use CTAD in template argument
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Keywords: rejects-valid
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: johelegp at gmail dot com
                CC: johelegp at gmail dot com
  Target Milestone: ---

See https://godbolt.org/z/P8xq8h9M1.

```C++
template<class T> struct X { T t; };
template<X> void f();
template<class T> void g() {
  f<X{T{0}}>();
}
int main() {
  g<int>();
}
```

```
<source>: In function 'void g()':
<source>:7:5: error: invalid use of 'X<...auto...>' in template argument
    7 |   f<X{T{0}}>();
      |     ^
Compiler returned: 1
```

[Bug c++/102933] Can't use CTAD in template argument

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102933

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
           Assignee|unassigned at gcc dot gnu.org      |ppalka at gcc dot gnu.org
   Last reconfirmed|                            |2021-10-26
   Target Milestone|---                         |11.3
             Status|UNCONFIRMED                 |ASSIGNED
                 CC|                            |ppalka at gcc dot gnu.org

[Bug c++/102933] Can't use CTAD in template argument

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102933

--- Comment #1 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Patrick Palka <ppalka@gcc.gnu.org>:

https://gcc.gnu.org/g:f70f17d03630fd948bc15a646ebe7e784125c4d4

commit r12-4772-gf70f17d03630fd948bc15a646ebe7e784125c4d4
Author: Patrick Palka <ppalka@redhat.com>
Date:   Thu Oct 28 10:46:46 2021 -0400

    c++: CTAD within template argument [PR102933]

    Here when checking for erroneous occurrences of 'auto' inside a template
    argument (which is allowed by the concepts TS for class templates),
    extract_autos_r picks up the CTAD placeholder for X{T{0}} which causes
    check_auto_in_tmpl_args to reject this valid template argument.  This
    patch fixes this by making extract_autos_r ignore CTAD placeholders.

    However, it seems we don't need to call check_auto_in_tmpl_args at all
    outside of the concepts TS since using 'auto' as a type-id is otherwise
    rejected more generally at parse time.  So this patch makes the function
    just exit early if !flag_concepts_ts.

    Similarly, I think the concepts code paths in do_auto_deduction and
    type_uses_auto are only necessary for the concepts TS, so this patch
    also restricts these code paths accordingly.

            PR c++/102933

    gcc/cp/ChangeLog:

            * parser.c (cp_parser_simple_type_specifier): Adjust diagnostic
            for using auto in parameter declaration.
            * pt.c (extract_autos_r): Ignore CTAD placeholders.
            (extract_autos): Use range-based for.
            (do_auto_deduction): Use extract_autos only for the concepts TS
            and not also for standard concepts.
            (type_uses_auto): Likewise with for_each_template_parm.
            (check_auto_in_tmpl_args): Just return false outside of the
            concepts TS.  Simplify.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp2a/nontype-class50.C: New test.
            * g++.dg/cpp2a/nontype-class50a.C: New test.

[Bug c++/102933] Can't use CTAD in template argument

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102933

--- Comment #2 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-11 branch has been updated by Patrick Palka
<ppalka@gcc.gnu.org>:

https://gcc.gnu.org/g:76c730cbf051281ff088ec18ee4f89fee4a8305a

commit r11-9387-g76c730cbf051281ff088ec18ee4f89fee4a8305a
Author: Patrick Palka <ppalka@redhat.com>
Date:   Thu Oct 28 10:46:46 2021 -0400

    c++: CTAD within template argument [PR102933]

    Here when checking for erroneous occurrences of 'auto' inside a template
    argument (which is allowed by the concepts TS for class templates),
    extract_autos_r picks up the CTAD placeholder for X{T{0}} which causes
    check_auto_in_tmpl_args to reject this valid template argument.  This
    patch fixes this by making extract_autos_r ignore CTAD placeholders.

    However, it seems we don't need to call check_auto_in_tmpl_args at all
    outside of the concepts TS since using 'auto' as a type-id is otherwise
    rejected more generally at parse time.  So this patch makes the function
    just exit early if !flag_concepts_ts.

    Similarly, I think the concepts code paths in do_auto_deduction and
    type_uses_auto are only necessary for the concepts TS, so this patch
    also restricts these code paths accordingly.

            PR c++/102933

    gcc/cp/ChangeLog:

            * parser.c (cp_parser_simple_type_specifier): Adjust diagnostic
            for using auto in parameter declaration.
            * pt.c (extract_autos_r): Ignore CTAD placeholders.
            (extract_autos): Use range-based for.
            (do_auto_deduction): Use extract_autos only for the concepts TS
            and not also for standard concepts.
            (type_uses_auto): Likewise with for_each_template_parm.
            (check_auto_in_tmpl_args): Just return false outside of the
            concepts TS.  Simplify.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp2a/nontype-class50.C: New test.
            * g++.dg/cpp2a/nontype-class50a.C: New test.

    (cherry picked from commit f70f17d03630fd948bc15a646ebe7e784125c4d4)

[Bug c++/102933] Can't use CTAD in template argument

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102933

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

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

--- Comment #3 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Fixed for GCC 11.3/12.