[Bug c++/110619] Dangling pointer returned from constexpr function converts in nullptr

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110619

--- Comment #6 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The trunk branch has been updated by Jason Merrill <jason@gcc.gnu.org>:

https://gcc.gnu.org/g:b8266af71c19a0bd7db4d08c8d2ee3c33214508c

commit r14-2772-gb8266af71c19a0bd7db4d08c8d2ee3c33214508c
Author: Nathaniel Shead <nathanieloshead@gmail.com>
Date:   Sun Jul 23 01:14:37 2023 +1000

    c++: Prevent dangling pointers from becoming nullptr in constexpr
[PR110619]

    Currently, when typeck discovers that a return statement will refer to a
    local variable it rewrites to return a null pointer. This causes the
    error messages for using the return value in a constant expression to be
    unhelpful, especially for reference return values, and is also a visible
    change to otherwise valid code (as in the linked PR).

    The transformation is nonetheless important, however, both as a safety
    guard against attackers being able to gain a handle to other data on the
    stack, and to prevent duplicate warnings from later null-dereference
    warning passes.

    As such, this patch just delays the transformation until cp_genericize,
    after constexpr function definitions have been generated.

            PR c++/110619

    gcc/cp/ChangeLog:

            * cp-gimplify.cc (cp_genericize_r): Transform RETURN_EXPRs to
            not return dangling pointers.
            * cp-tree.h (RETURN_EXPR_LOCAL_ADDR_P): New flag.
            (check_return_expr): Add a new parameter.
            * semantics.cc (finish_return_stmt): Set flag on RETURN_EXPR
            when referring to dangling pointer.
            * typeck.cc (check_return_expr): Disable transformation of
            dangling pointers, instead pass this information to caller.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp1y/constexpr-110619.C: New test.

    Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>