[Bug c++/100493] Lambda default copy capture that captures "this" cannot be used in both C++17 and C++20 modes

["cvs-commit at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100493

--- Comment #6 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-11 branch has been updated by Patrick Palka
<ppalka@gcc.gnu.org>:

https://gcc.gnu.org/g:d33f68865f6f71c28cfee5dfd91765e86d471ef1

commit r11-9391-gd33f68865f6f71c28cfee5dfd91765e86d471ef1
Author: Patrick Palka <ppalka@redhat.com>
Date:   Mon Nov 29 07:52:47 2021 -0500

    c++: redundant explicit 'this' capture before C++20 [PR100493]

    As described in detail in the PR, in C++20 implicitly capturing 'this'
    via a '=' capture default is deprecated, and in C++17 adding an explicit
    'this' capture alongside a '=' capture default is diagnosed as redundant
    (and is strictly speaking ill-formed).  This means it's impossible to
    write, in a forward-compatible way, a C++17 lambda that has a '=' capture
    default and that also captures 'this' (implicitly or explicitly):

      [=] { this; }      // #1 deprecated in C++20, OK in C++17
                         // GCC issues a -Wdeprecated warning in C++20 mode

      [=, this] { }      // #2 ill-formed in C++17, OK in C++20
                         // GCC issues an unconditional warning in C++17 mode

    This patch resolves this dilemma by downgrading the warning for #2 into
    a -pedantic one.  In passing, move it into the -Wc++20-extensions class
    of warnings and adjust its wording accordingly.

            PR c++/100493

    gcc/cp/ChangeLog:

            * parser.c (cp_parser_lambda_introducer): In C++17, don't
            diagnose a redundant 'this' capture alongside a by-copy
            capture default unless -pedantic.  Move the diagnostic into
            -Wc++20-extensions and adjust wording accordingly.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp1z/lambda-this1.C: Adjust expected diagnostics.
            * g++.dg/cpp1z/lambda-this8.C: New test.
            * g++.dg/cpp2a/lambda-this3.C: Compile with -pedantic in C++17
            to continue to diagnose redundant 'this' captures.

    (cherry picked from commit 1420ff3efcff98df0e8c6f021a7ff24b5fc65043)