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 >