GCC rejects: constexpr const int *p = nullptr; constexpr int *q = const_cast<int*>(p); saying: <source>:2:20: error: conversion of 'const int*' null pointer to 'int*' is not a constant expression 2 | constexpr int *q = const_cast<int*>(p); | ^~~~~~~~~~~~~~~~~~~ I don't think any such rule exists, and other compilers accept. This only appears to affect const_casts of null pointers; non-null pointer const casts seem to work OK. Perhaps GCC thinks that this is a reinterpret_cast / cast from void* or something like that? It looks like this regressed between GCC 6 and GCC 7.
Thanks for the report. Started with r238909.
I'll take a look.
Same problem for: constexpr int* ptr = (int *) (const int *) nullptr;
The master branch has been updated by Marek Polacek <mpolacek@gcc.gnu.org>: https://gcc.gnu.org/g:2ffc26458dd7ba7b3fa00897f2d8c6cd24ba06f3 commit r11-7404-g2ffc26458dd7ba7b3fa00897f2d8c6cd24ba06f3 Author: Marek Polacek <polacek@redhat.com> Date: Wed Feb 24 10:08:44 2021 -0500 c++: const_cast of null pointer in constant expr [PR99176] Here we reject constexpr const int *p = nullptr; constexpr int *q = const_cast<int*>(p); with "conversion of 'const int*' null pointer to 'int*' is not a constant expression", which seems bogus. This code has been rejected since r238909 which added the can_convert check when converting a null pointer. I'm not finding any standard rule that this check was supposed to enforce. The original discussion was here <https://gcc.gnu.org/legacy-ml/gcc-patches/2016-06/msg01447.html> and here <https://gcc.gnu.org/legacy-ml/gcc-patches/2016-07/msg00280.html>. Since can_convert never assumes a C-style cast, it rejects casting away constness as in the test above and in: constexpr int *q = (int *)(const int *) nullptr; Removing the check only breaks constexpr-nullptr-2.C by not giving any diagnostic for line 229: constexpr B *pb2 = static_cast<B*>(pa0); // { dg-error "not a constant expression" } but the cast seems to be valid: we do [expr.static.cast]/7, and [expr.const] only says that a reinterpreter_cast and converting from void* is invalid in constexpr. The can_convert check rejected convering from void *, but only when converting from a null pointer, so it's not good enough. So I've added a check to catch conversions from cv void*. I realize it's not a great time to be adding additional checking, but removing the can_convert check would then technically be a regression. Let's limit the new check to only trigger for integer_zerop and then remove it in GCC 12. gcc/cp/ChangeLog: DR 1312 PR c++/99176 * constexpr.c (is_std_construct_at): New overload. (is_std_allocator_allocate): New overload. (cxx_eval_call_expression): Use the new overloads. (cxx_eval_constant_expression): Reject casting from void * as per DR 1312. Don't check can_convert. gcc/testsuite/ChangeLog: DR 1312 PR c++/99176 * g++.dg/cpp0x/constexpr-nullptr-2.C: Adjust dg-error. * g++.dg/cpp0x/constexpr-cast2.C: New test. * g++.dg/cpp0x/constexpr-cast3.C: New test.
Fixed on trunk so far.
The releases/gcc-10 branch has been updated by Marek Polacek <mpolacek@gcc.gnu.org>: https://gcc.gnu.org/g:581e87b67233556d566df3a0ea33be9c19fbcb2f commit r10-9687-g581e87b67233556d566df3a0ea33be9c19fbcb2f Author: Marek Polacek <polacek@redhat.com> Date: Fri Apr 9 18:41:21 2021 -0400 c++: const_cast of null pointer in constant expr [PR99176] Here we reject constexpr const int *p = nullptr; constexpr int *q = const_cast<int*>(p); with "conversion of 'const int*' null pointer to 'int*' is not a constant expression", which seems bogus. This code has been rejected since r238909 which added the can_convert check when converting a null pointer. I'm not finding any standard rule that this check was supposed to enforce. The original discussion was here <https://gcc.gnu.org/legacy-ml/gcc-patches/2016-06/msg01447.html> and here <https://gcc.gnu.org/legacy-ml/gcc-patches/2016-07/msg00280.html>. Since can_convert never assumes a C-style cast, it rejects casting away constness as in the test above and in: constexpr int *q = (int *)(const int *) nullptr; Removing the check only breaks constexpr-nullptr-2.C by not giving any diagnostic for line 229: constexpr B *pb2 = static_cast<B*>(pa0); // { dg-error "not a constant expression" } but the cast seems to be valid: we do [expr.static.cast]/7, and [expr.const] only says that a reinterpreter_cast and converting from void* is invalid in constexpr. The can_convert check rejected convering from void *, but only when converting from a null pointer, so it's not good enough. So I've added a check to catch conversions from cv void*. I realize it's not a great time to be adding additional checking, but removing the can_convert check would then technically be a regression. Let's limit the new check to only trigger for integer_zerop and then remove it in GCC 12. gcc/cp/ChangeLog: DR 1312 PR c++/99176 * constexpr.c (is_std_construct_at): New overload. (is_std_allocator_allocate): New overload. (cxx_eval_call_expression): Use the new overloads. (cxx_eval_constant_expression): Reject casting from void * as per DR 1312. Don't check can_convert. gcc/testsuite/ChangeLog: DR 1312 PR c++/99176 * g++.dg/cpp0x/constexpr-nullptr-2.C: Adjust dg-error. * g++.dg/cpp0x/constexpr-cast2.C: New test. * g++.dg/cpp0x/constexpr-cast3.C: New test.
Fixed in GCC 10.4 too.