I noticed this patchset did not see any comment, did not get committed,
and the PR is still open, so let me loop in Jason and Nathan as C++
maintainers.
Ed, looking at the ChangeLog something seems missing:
gcc/cp/ChangeLog:
* call.c (tourney):
(joust):
(build_user_type_conversion_1):
(reference_binding):
(implicit_conversion_1):
(build_user_type_conversion):
:
There should be a description of what changed after the colons. Can you
send an updated patch?
Gerald
On Thu, 30 Dec 2021, Ed Catmur wrote:
> This patch series improves conformance to the C++ standard for C-style casts
> (explicit cast notation, [expr.cast]) in two scenarios.
>
> First, as discussed in CWG 909 [1], a C-style cast may invoke a user-defined
> conversion (as a static_cast) followed by a const_cast; currently this is
> erroneously rejected, tracked in PR77465 [2].
>
> Second, a C-style cast to non-const reference to arithmetic type can and thus
> should be interpreted as a static_cast reference binding to temporary
> followed by a const_cast; currently it is interpreted as a reinterpret_cast
> (possibly followed by const_cast). To the best of my knowledge this has not
> been reported as a defect; credit to Turtlefight on Stack Overflow [3] for
> analyzing the situation, drawing my attention to the above issues, and
> convincing me that my interpretation was correct. Note that C-style cast to
> /const/ arithmetic reference will already introduce a temporary; this patch
> only changes the behavior where the reference is to non-const.
>
> Clearly there is the potential for miscompilation of existing code that
> assumes an incorrect interpretation (reinterpret_cast); a number of existing
> test cases are amended in this patch series. In consideration of this, I have
> checked bootstrap (on x86_64) with no new errors, and add a warning (to
> Wconversion) where a C-style cast to arithmetic reference introduces a
> temporary in a situation where the user may expect a reinterpret_cast
> (type-punning); this affects both the const (existing) and non-const case.
> Note that this change will not affect C code compiled as C++, since that code
> cannot contain user-defined conversions or references.
>
> In this patch series,
> * 1/3 - allow C-style cast to invoke user-defined conversion followed by
> const_cast, resolving PR77465; adds a test case and fixes existing test cases
> which expect this to be rejected
> * 2/3 - allow C-style cast to non-const reference to arithmetic type to
> invoke static_cast reference binding to temporary followed by const_cast;
> adds a test case and fixes existing test cases which expect reinterpret_cast
> semantics
> * 3/3 - add comprehensive test suite covering expr.cast scenarios, positive
> and negative.
>
> 1. http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_closed.html#909
> 2. https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77465
> 3. https://stackoverflow.com/a/70400230/567292
>
