The proposed resolution to CWG 2563 clarifies that any conversions to be applied between the result of promise.get_return_object() and the ramp return object should be carried out as part of the return expression (rather than at the time promise.get_return_object() is evaluated). It also makes clear that, when the two types are the same, we can treat this as if the promise.get_return_object() temp was constructed in the return slot.
In addition, the resolution proposes that the type of the temporary object used to hold the result of promise.get_return_object() should be `decltype(auto)`. The patch series starts with two prerequisites (the first is general). 1. When there is only one BLOCK in a function (which can happen with coroutine lambda ramps), we need to make sure that this outer BLOCK is marked as the outer brace one. 2. We must allow NVRO in more cases for ramp functions to allow us to create non-movable/copyable objects in the g_r_o and then return them. 3. This reworks the promise.get_return_object()/return sequences so that we only omit the temporary when the coroutine ramp is void. We also switch to using a regular cleanup for the g_r_o since that is ramp-local. This also returns GCC to the GCC-14 behaviour in this respect. 4. This implements the CWG2563 change to the type of the g_r_o. It is a behaviour change when promise.get_return_object() returns a reference (although so far no tests I have tried seem to depend on the existing behaviour - possibly because that would require some effort to make visible). 5. Rather than handling the cleanups via a try-catch block, this switches to using eh-only cleanup statements (which reduces the number of 'live' flags we need to create and maintain). The patches have been tested incrementally on x86_64, powerpc64le, aarch64 linux, sparc9 solaris, x86_64 darwin. OK for trunk? and after some bake time 15? thanks Iain ===== Iain Sandoe (5): c++: Set the outer brace marker for missed cases. c++, coroutines: Allow NVRO in more cases for ramp functions. c++, coroutines: Address CWG2563 return value init [PR119916]. c++, coroutines: Use decltype(auto) for the g_r_o. c++, coroutines: Clean up the ramp cleanups. gcc/cp/coroutines.cc | 331 ++++++------------ gcc/cp/decl.cc | 8 +- gcc/cp/typeck.cc | 3 + gcc/testsuite/g++.dg/coroutines/pr115908.C | 69 ++-- .../g++.dg/coroutines/torture/pr119916.C | 66 ++++ .../special-termination-00-sync-completion.C | 2 +- .../special-termination-01-self-destruct.C | 2 +- 7 files changed, 231 insertions(+), 250 deletions(-) create mode 100644 gcc/testsuite/g++.dg/coroutines/torture/pr119916.C -- 2.39.2 (Apple Git-143)
