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

[eric.niebler at gmail dot com via Gcc-bugs]

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.