[Bug c++/122688] New: Lambda returning a local type rejected in decltype

[pdimov at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=122688

            Bug ID: 122688
           Summary: Lambda returning a local type rejected in decltype
           Product: gcc
           Version: 16.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pdimov at gmail dot com
  Target Milestone: ---

The following code:

using R = decltype([]{ struct X{}; return X{}; }());

is rejected by GCC with

error: types may not be defined in 'decltype' expressions

but it works under MSVC and Clang (in C++20 mode and above.)