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.

3 files changed, 50 insertions, 5 deletions

diff --git a/gcc/cp/typeck.cc b/gcc/cp/typeck.cc
index 6c91182..281d281 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 0000000..e80834c
--- /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 c548587..8f48ba8 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>);