[Bug c++/103969] New: 'auto' parameter not permitted in this context

[egor.pugin at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103969

            Bug ID: 103969
           Summary: 'auto' parameter not permitted in this context
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: egor.pugin at gmail dot com
  Target Milestone: ---

template <auto = [](auto &&v) -> decltype(auto) { return v; }>
struct s {};

gives

Output of x86-64 gcc (trunk) (Compiler #2)
<source>:1:21: error: 'auto' parameter not permitted in this context
    1 | template <auto = [](auto &&v) -> decltype(auto) { return v; }>
      |                     ^~~~
<source>: In lambda function:
<source>:1:58: error: 'v' was not declared in this scope
    1 | template <auto = [](auto &&v) -> decltype(auto) { return v; }>
      |                                                          ^


clang, msvc work fine.
https://godbolt.org/z/4Mf5efeWY