[gcc r13-4097] c++: Alignment changes to layout compatibility/common initial sequence - DR2583

[Jakub Jelinek <jakub@gcc.gnu.org>]

https://gcc.gnu.org/g:c5e8c6c193d9a1ab80ef1a05e5158ddf35d54009

commit r13-4097-gc5e8c6c193d9a1ab80ef1a05e5158ddf35d54009
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Wed Nov 16 14:49:47 2022 +0100

    c++: Alignment changes to layout compatibility/common initial sequence - DR2583
    
    When trying to figure out what to do about alignment,
    layout_compatible_type_p returns false if TYPE_ALIGN on
    ENUMERAL_TYPE/CLASS_TYPE_P (but not scalar types?) differ, or if members
    don't have the same positions.
    
    What is in DR2583 doesn't say anything like that though, on the other side
    it says that if the corresponding entities don't have the same alignment
    requirements, they aren't part of the common initial sequence.
    
    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.
    
    2022-11-16  Jakub Jelinek  <jakub@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.

Diff:
---
 gcc/cp/typeck.cc                                   |  6 +--
 gcc/testsuite/g++.dg/DRs/dr2583.C                  | 45 ++++++++++++++++++++++
 gcc/testsuite/g++.dg/cpp2a/is-layout-compatible3.C |  4 +-
 3 files changed, 50 insertions(+), 5 deletions(-)

diff --git a/gcc/cp/typeck.cc b/gcc/cp/typeck.cc
index 6c911827778..281d281629b 100644
--- a/gcc/cp/typeck.cc
+++ b/gcc/cp/typeck.cc
@@ -1833,6 +1833,8 @@ next_common_initial_sequence (tree &memb1, tree &memb2)
   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, tree type2)
   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);
diff --git a/gcc/testsuite/g++.dg/DRs/dr2583.C b/gcc/testsuite/g++.dg/DRs/dr2583.C
new file mode 100644
index 00000000000..e80834c6215
--- /dev/null
+++ b/gcc/testsuite/g++.dg/DRs/dr2583.C
@@ -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
diff --git a/gcc/testsuite/g++.dg/cpp2a/is-layout-compatible3.C b/gcc/testsuite/g++.dg/cpp2a/is-layout-compatible3.C
index c5485874660..8f48ba8679a 100644
--- a/gcc/testsuite/g++.dg/cpp2a/is-layout-compatible3.C
+++ b/gcc/testsuite/g++.dg/cpp2a/is-layout-compatible3.C
@@ -55,10 +55,10 @@ static_assert (!std::is_layout_compatible_v<K, L>);
 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>);