GCC rejects following code: int* (*xx)[]; const int* const(*yy)[]; yy = xx; while Clang started accepting since 7.0 when they implemented CWG 330. Without this it is very hard to implement std::span, as its constructors availability are defined in terms of pointer to array convertibility (see [span.cons]).
Confirmed. [Moved to DR as N4261 at the November, 2014 meeting.] The resolution is in https://wg21.link/n4261
Actually, even current C++17 unique_ptr::reset for array objects is defined in terms of pointer to array convertibility ([unique.ptr.runtime.modifiers]).
I see that std::span implementation was proposed in https://gcc.gnu.org/ml/libstdc++/2019-08/msg00068.html. Was this bug fixed for implenting it? Is it possible to make std::span<const T> from std::vector<T>?
(In reply to Viktor Ostashevskyi from comment #3) > Was this bug fixed for implenting it? No. > Is it possible to make std::span<const T> from std::vector<T>? Yes, that works with current trunk.
(In reply to Jonathan Wakely from comment #4) > (In reply to Viktor Ostashevskyi from comment #3) > > Is it possible to make std::span<const T> from std::vector<T>? > > Yes, that works with current trunk. Although that could be a bug: the constraints use declval<_Container&>() but should be using declval<const _Container&>().
I think I have a patch; e.g. this now compiles: double *const (*d)[3]; double const *const (*e)[3] = d; int *(*f)[3]; const int *const (*g)[3] = f; int main() { double *array2D[2][3]; double * (*array2DPtr1)[3] = array2D; // Legal double * const (*array2DPtr2)[3] = array2DPtr1; // Legal double const * const (*array2DPtr3)[3] = array2DPtr2; // Illegal }
Author: mpolacek Date: Wed Oct 9 20:58:00 2019 New Revision: 276771 URL: https://gcc.gnu.org/viewcvs?rev=276771&root=gcc&view=rev Log: Implement C++20 P0388R4, DR 1307, and DR 330. This patch implements P0388R4, Permit conversions to arrays of unknown bound, <http://wg21.link/p0388r4>. CWG 393 allowed references to arrays of unknown bound and this C++20 feature allows conversions like void f(int(&)[]); int arr[1]; void g() { f(arr); } int(&r)[] = arr; The proposal seemed fairly straightforward but it turned out to be quite shifty. I found out that I needed to implement DR 2352 (done), and also DR 1307 (done in this patch). The latter DR added wording for list-initialization ranking of references to arrays which this proposal extends. DR 330 was also implemented in this patch. PR c++/91364 - P0388R4: Permit conversions to arrays of unknown bound. PR c++/69531 - DR 1307: Differently bounded array parameters. PR c++/88128 - DR 330: Qual convs and pointers to arrays of pointers. * call.c (build_array_conv): Build ck_identity at the beginning of the conversion. (standard_conversion): Pass bounds_none to comp_ptr_ttypes_const. (maybe_warn_array_conv): New. (convert_like_real): Call it. Add an error message about converting from arrays of unknown bounds. (conv_get_original_expr): New. (nelts_initialized_by_list_init): New. (conv_binds_to_array_of_unknown_bound): New. (compare_ics): Implement list-initialization ranking based on array sizes, as specified in DR 1307 and P0388R. * cp-tree.h (comp_ptr_ttypes_const): Adjust declaration. (compare_bounds_t): New enum. * typeck.c (comp_array_types): New bool and compare_bounds_t parameters. Use them. (structural_comptypes): Adjust the call to comp_array_types. (similar_type_p): Handle ARRAY_TYPE. (build_const_cast_1): Pass bounds_none to comp_ptr_ttypes_const. (comp_ptr_ttypes_real): Don't check cv-quals of ARRAY_TYPEs. Use comp_array_types to compare array types. Look through arrays as per DR 330. (comp_ptr_ttypes_const): Use comp_array_types to compare array types. Look through arrays as per DR 330. * g++.dg/conversion/qual1.C: New test. * g++.dg/conversion/qual2.C: New test. * g++.dg/conversion/qual3.C: New test. * g++.dg/conversion/ref2.C: New test. * g++.dg/conversion/ref3.C: New test. * g++.dg/cpp0x/initlist-array3.C: Remove dg-error. * g++.dg/cpp0x/initlist-array7.C: New test. * g++.dg/cpp0x/initlist-array8.C: New test. * g++.dg/cpp2a/array-conv1.C: New test. * g++.dg/cpp2a/array-conv10.C: New test. * g++.dg/cpp2a/array-conv11.C: New test. * g++.dg/cpp2a/array-conv12.C: New test. * g++.dg/cpp2a/array-conv13.C: New test. * g++.dg/cpp2a/array-conv14.C: New test. * g++.dg/cpp2a/array-conv15.C: New test. * g++.dg/cpp2a/array-conv16.C: New test. * g++.dg/cpp2a/array-conv17.C: New test. * g++.dg/cpp2a/array-conv2.C: New test. * g++.dg/cpp2a/array-conv3.C: New test. * g++.dg/cpp2a/array-conv4.C: New test. * g++.dg/cpp2a/array-conv5.C: New test. * g++.dg/cpp2a/array-conv6.C: New test. * g++.dg/cpp2a/array-conv7.C: New test. * g++.dg/cpp2a/array-conv8.C: New test. * g++.dg/cpp2a/array-conv9.C: New test. * g++.old-deja/g++.bugs/900321_01.C: Adjust dg-error. * testsuite/23_containers/span/lwg3255.cc: Adjust test to match the post-P0388R4 behavior. Added: trunk/gcc/testsuite/g++.dg/conversion/qual1.C trunk/gcc/testsuite/g++.dg/conversion/qual2.C trunk/gcc/testsuite/g++.dg/conversion/qual3.C trunk/gcc/testsuite/g++.dg/conversion/ref2.C trunk/gcc/testsuite/g++.dg/conversion/ref3.C trunk/gcc/testsuite/g++.dg/cpp0x/initlist-array7.C trunk/gcc/testsuite/g++.dg/cpp0x/initlist-array8.C trunk/gcc/testsuite/g++.dg/cpp2a/array-conv1.C trunk/gcc/testsuite/g++.dg/cpp2a/array-conv10.C trunk/gcc/testsuite/g++.dg/cpp2a/array-conv11.C trunk/gcc/testsuite/g++.dg/cpp2a/array-conv12.C trunk/gcc/testsuite/g++.dg/cpp2a/array-conv13.C trunk/gcc/testsuite/g++.dg/cpp2a/array-conv14.C trunk/gcc/testsuite/g++.dg/cpp2a/array-conv15.C trunk/gcc/testsuite/g++.dg/cpp2a/array-conv16.C trunk/gcc/testsuite/g++.dg/cpp2a/array-conv17.C trunk/gcc/testsuite/g++.dg/cpp2a/array-conv2.C trunk/gcc/testsuite/g++.dg/cpp2a/array-conv3.C trunk/gcc/testsuite/g++.dg/cpp2a/array-conv4.C trunk/gcc/testsuite/g++.dg/cpp2a/array-conv5.C trunk/gcc/testsuite/g++.dg/cpp2a/array-conv6.C trunk/gcc/testsuite/g++.dg/cpp2a/array-conv7.C trunk/gcc/testsuite/g++.dg/cpp2a/array-conv8.C trunk/gcc/testsuite/g++.dg/cpp2a/array-conv9.C Modified: trunk/gcc/cp/ChangeLog trunk/gcc/cp/call.c trunk/gcc/cp/cp-tree.h trunk/gcc/cp/typeck.c trunk/gcc/testsuite/ChangeLog trunk/gcc/testsuite/g++.dg/cpp0x/initlist-array3.C trunk/gcc/testsuite/g++.old-deja/g++.bugs/900321_01.C trunk/libstdc++-v3/ChangeLog trunk/libstdc++-v3/testsuite/23_containers/span/lwg3255.cc
Implemented for GCC 10.