On Tue, Nov 15, 2022 at 05:57:26PM -0500, Jason Merrill wrote: > > So, my understanding of this is we shouldn't check TYPE_ALIGN in > > layout_compatible_type_p, but instead DECL_ALIGN in > > next_common_initial_seqence. > > Agreed. > > > +#if __cpp_lib_is_layout_compatible >= 201907L > > +static_assert (std::is_corresponding_member (&A::i, &B::i), ""); > > +static_assert (alignof (char) == 8 || !std::is_corresponding_member > > (&A::c, &B::c), ""); > > Maybe > > std_is_corresponding_member (&A::c, &B::c) == (alignof (char) == 8) > > ? > > Could also use an alignas(alignof(type)) case to verify that it matches a > member with no alignas.
So like this? The previous patch successfully bootstrapped/regtested on x86_64-linux and i686-linux, only the new testcase changed and that one passes make check, ok for trunk? 2022-11-16 Jakub Jelinek <ja...@redhat.com> * typeck.cc (next_common_initial_sequence): Return false members have different DECL_ALIGN. (layout_compatible_type_p): Don't test TYPE_ALIGN of ENUMERAL_TYPE or CLASS_TYPE_P. * g++.dg/cpp2a/is-layout-compatible3.C: Expect enums with different alignas to be layout compatible, while classes with different alignas on members layout incompatible. * g++.dg/DRs/dr2583.C: New test. --- gcc/cp/typeck.cc.jj 2022-11-13 04:53:46.010682269 -1200 +++ gcc/cp/typeck.cc 2022-11-13 23:14:41.355180354 -1200 @@ -1833,6 +1833,8 @@ next_common_initial_sequence (tree &memb1 if ((!lookup_attribute ("no_unique_address", DECL_ATTRIBUTES (memb1))) != !lookup_attribute ("no_unique_address", DECL_ATTRIBUTES (memb2))) return false; + if (DECL_ALIGN (memb1) != DECL_ALIGN (memb2)) + return false; if (!tree_int_cst_equal (bit_position (memb1), bit_position (memb2))) return false; return true; @@ -1854,15 +1856,13 @@ layout_compatible_type_p (tree type1, tr type2 = cp_build_qualified_type (type2, TYPE_UNQUALIFIED); if (TREE_CODE (type1) == ENUMERAL_TYPE) - return (TYPE_ALIGN (type1) == TYPE_ALIGN (type2) - && tree_int_cst_equal (TYPE_SIZE (type1), TYPE_SIZE (type2)) + return (tree_int_cst_equal (TYPE_SIZE (type1), TYPE_SIZE (type2)) && same_type_p (finish_underlying_type (type1), finish_underlying_type (type2))); if (CLASS_TYPE_P (type1) && std_layout_type_p (type1) && std_layout_type_p (type2) - && TYPE_ALIGN (type1) == TYPE_ALIGN (type2) && tree_int_cst_equal (TYPE_SIZE (type1), TYPE_SIZE (type2))) { tree field1 = TYPE_FIELDS (type1); --- gcc/testsuite/g++.dg/cpp2a/is-layout-compatible3.C.jj 2021-08-18 21:42:27.414421719 -1200 +++ gcc/testsuite/g++.dg/cpp2a/is-layout-compatible3.C 2022-11-13 23:20:05.008776825 -1200 @@ -55,10 +55,10 @@ static_assert (!std::is_layout_compatibl static_assert (!std::is_layout_compatible_v<M, N>); static_assert (!std::is_layout_compatible_v<O, P>); static_assert (!std::is_layout_compatible_v<P, D>); -static_assert (!std::is_layout_compatible_v<Q, R>); +static_assert (std::is_layout_compatible_v<Q, R>); static_assert (!std::is_layout_compatible_v<U, V>); static_assert (!std::is_layout_compatible_v<A, I>); static_assert (!std::is_layout_compatible_v<C, I>); -static_assert (std::is_layout_compatible_v<E, F>); +static_assert (!std::is_layout_compatible_v<E, F>); static_assert (std::is_layout_compatible_v<G, H>); static_assert (std::is_layout_compatible_v<C1, D1>); --- gcc/testsuite/g++.dg/DRs/dr2583.C.jj 2022-11-16 11:28:03.200831332 +0100 +++ gcc/testsuite/g++.dg/DRs/dr2583.C 2022-11-16 11:31:34.421978999 +0100 @@ -0,0 +1,45 @@ +// DR 2583 - Common initial sequence should consider over-alignment. +// { dg-do compile { target c++11 } } + +#include <type_traits> + +struct A { + int i; + char c; +}; + +struct B { + int i; + alignas(8) char c; +}; + +struct C { + int i; + alignas(alignof(char)) char c; +}; + +struct D { + alignas(alignof(int)) int i; + char c; +}; + +struct S0 { + alignas(16) char x[128]; + int i; +}; + +struct alignas(16) S1 { + char x[128]; + int i; +}; + +#if __cpp_lib_is_layout_compatible >= 201907L +static_assert (std::is_corresponding_member (&A::i, &B::i), ""); +static_assert (std::is_corresponding_member (&A::c, &B::c) == (alignof (char) == 8), ""); +static_assert (std::is_corresponding_member (&A::i, &C::i), ""); +static_assert (std::is_corresponding_member (&A::c, &C::c), ""); +static_assert (std::is_corresponding_member (&A::i, &D::i), ""); +static_assert (std::is_corresponding_member (&A::c, &D::c), ""); +static_assert (std::is_corresponding_member (&S0::x, &S1::x) == (alignof (char) == 16), ""); +static_assert (std::is_corresponding_member (&S0::i, &S1::i) == (alignof (char) == 16), ""); +#endif Jakub