[Bug c++/109781] New: erroneous error "lambda-expression in template parameter type" for tparam lambda returning a lambda

[Bug c++/109781] New: erroneous error "lambda-expression in template parameter type" for tparam lambda returning a lambda

[eric.niebler at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109781

            Bug ID: 109781
           Summary: erroneous error "lambda-expression in template
                    parameter type" for tparam lambda returning a lambda
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: eric.niebler at gmail dot com
  Target Milestone: ---

template <auto A = []{ return []{}; }>
struct C {};


... on trunk with `-std=c++20` results in:

<source>: In lambda function:
<source>:1:31: error: lambda-expression in template parameter type
    1 | template <auto A = []{ return []{}; }>
      |                               ^
Compiler returned: 1


I don't know of a reason why this code should be rejected.

[Bug c++/109781] erroneous error "lambda-expression in template parameter type" for tparam lambda returning a lambda

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109781

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2023-05-08
             Status|UNCONFIRMED                 |NEW
           Keywords|                            |rejects-valid

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Confirmed.

[Bug c++/109781] [11/12/13/14 Regression] erroneous error "lambda-expression in template parameter type" for tparam lambda returning a lambda

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109781

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |11.4
            Summary|erroneous error             |[11/12/13/14 Regression]
                   |"lambda-expression in       |erroneous error
                   |template parameter type"    |"lambda-expression in
                   |for tparam lambda returning |template parameter type"
                   |a lambda                    |for tparam lambda returning
                   |                            |a lambda
      Known to work|                            |10.3.0
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=99478
                 CC|                            |jason at gcc dot gnu.org,
                   |                            |ppalka at gcc dot gnu.org

--- Comment #2 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Started with r11-8166-ge1666ebd9ad31d.

[Bug c++/109781] [11/12/13/14 Regression] erroneous error "lambda-expression in template parameter type" for tparam lambda returning a lambda

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109781

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|11.4                        |11.5

--- Comment #3 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
GCC 11.4 is being released, retargeting bugs to GCC 11.5.

[Bug c++/109781] [11/12/13/14 Regression] erroneous error "lambda-expression in template parameter type" for tparam lambda returning a lambda

[johelegp at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109781

--- Comment #4 from Johel Ernesto Guerrero Peña <johelegp at gmail dot com> ---
This isn't valid since <https://cplusplus.github.io/CWG/issues/2542.html>.

[Bug c++/109781] [11/12/13/14 Regression] erroneous error "lambda-expression in template parameter type" for tparam lambda returning a lambda

[johelegp at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109781

--- Comment #5 from Johel Ernesto Guerrero Peña <johelegp at gmail dot com> ---
Add `+` to convert it to a pointer,
and it's still rejected (<https://compiler-explorer.com/z/PMras4rzd>):

```C++
template <auto A = +[]{ return []{}; }>
struct C {};
```

```output
<source>: In lambda function:
<source>:1:32: error: lambda-expression in template parameter type
    1 | template <auto A = +[]{ return []{}; }>
      |                                ^
Compiler returned: 1
```

[Bug c++/109781] [11/12/13/14 Regression] erroneous error "lambda-expression in template parameter type" for tparam lambda returning a lambda

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109781

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Priority|P3                          |P2

--- Comment #6 from Richard Biener <rguenth at gcc dot gnu.org> ---
I wonder if it actually "works" in GCC 10?