Bug 102933 - Can't use CTAD in template argument
Summary: Can't use CTAD in template argument
Status: ASSIGNED
Alias: None
Product: gcc
Classification: Unclassified
Component: c++ (show other bugs)
Version: 12.0
: P3 normal
Target Milestone: 11.3
Assignee: Patrick Palka
URL:
Keywords: rejects-valid
Depends on:
Blocks:
 
Reported: 2021-10-25 17:11 UTC by Johel Ernesto Guerrero Peña
Modified: 2021-10-28 14:46 UTC (History)
3 users (show)

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Known to work:
Known to fail:
Last reconfirmed: 2021-10-26 00:00:00


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Description Johel Ernesto Guerrero Peña 2021-10-25 17:11:22 UTC
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
```
Comment 1 CVS Commits 2021-10-28 14:46:59 UTC
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.