[Bug c++/104304] New: Compiler allows using a lambda with a capture as non-type template parameter (C++20)

[dk949.david at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104304

            Bug ID: 104304
           Summary: Compiler allows using a lambda with a capture as
                    non-type template parameter (C++20)
           Product: gcc
           Version: 11.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: dk949.david at gmail dot com
  Target Milestone: ---

Created attachment 52316
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=52316&action=edit
Small test case (preprocessed .ii).

GCC version: 11.1.0
System type: linux x86_64


Flags used to compile the test case: '-Wpedantic' '-Wall' '-Wextra'
'-std=c++20'
Compiler output: No output


Expected behavior: Compilation error, or a warning.
Observed behavior: Sample compiles with no warnings or errors.

Details:
It should not be possible (if I understand the standard correctly) to pass a
lambda which captures something as a non-type template parameter, as it is not
a structured* type (because it has private non-static** members). 


* C++ ISO standard 13.2 (6)
** C++ ISO standard 7.5.5.3 (10.2)