[gcc r16-5516] c++: Correct behaviour of layout_compatible_type for aligned types

[Nathaniel Shead]

https://gcc.gnu.org/g:59cf3fa79caa945a99fdc3b5bd349987fd23dd92

commit r16-5516-g59cf3fa79caa945a99fdc3b5bd349987fd23dd92
Author: Nathaniel Shead <nathanieloshead@gmail.com>
Date:   Sat Nov 22 22:11:35 2025 +1100

    c++: Correct behaviour of layout_compatible_type for aligned types
    
    The standard does not require two types to have the same alignment (and
    hence size) to be considered layout-compatible.  The same applies to
    members of unions.
    
    gcc/cp/ChangeLog:
    
            * typeck.cc (layout_compatible_type_p): Do not check TYPE_SIZE.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/cpp2a/is-layout-compatible3.C: Adjust expected results.
    
    Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
    Reviewed-by: Jason Merrill <jason@redhat.com>

Diff:
---
 gcc/cp/typeck.cc                                   | 8 +++-----
 gcc/testsuite/g++.dg/cpp2a/is-layout-compatible3.C | 8 ++++----
 2 files changed, 7 insertions(+), 9 deletions(-)

diff --git a/gcc/cp/typeck.cc b/gcc/cp/typeck.cc
index 2ab45f3fff6e..0b8ef84bf50a 100644
--- a/gcc/cp/typeck.cc
+++ b/gcc/cp/typeck.cc
@@ -1883,14 +1883,12 @@ layout_compatible_type_p (tree type1, tree type2)
   type2 = cp_build_qualified_type (type2, TYPE_UNQUALIFIED);
 
   if (TREE_CODE (type1) == ENUMERAL_TYPE)
-    return (tree_int_cst_equal (TYPE_SIZE (type1), TYPE_SIZE (type2))
-	    && same_type_p (finish_underlying_type (type1),
-			    finish_underlying_type (type2)));
+    return 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)
-      && tree_int_cst_equal (TYPE_SIZE (type1), TYPE_SIZE (type2)))
+      && std_layout_type_p (type2))
     {
       tree field1 = TYPE_FIELDS (type1);
       tree field2 = TYPE_FIELDS (type2);
diff --git a/gcc/testsuite/g++.dg/cpp2a/is-layout-compatible3.C b/gcc/testsuite/g++.dg/cpp2a/is-layout-compatible3.C
index 499ba4977d41..e48679fef11f 100644
--- a/gcc/testsuite/g++.dg/cpp2a/is-layout-compatible3.C
+++ b/gcc/testsuite/g++.dg/cpp2a/is-layout-compatible3.C
@@ -51,15 +51,15 @@ struct B1 { signed int b; };
 struct alignas (16) C1 : public A1 {};
 struct alignas (16) D1 : public B1 {};
 
-static_assert (!std::is_layout_compatible_v<I, J>);
+static_assert (std::is_layout_compatible_v<I, J>);
 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<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<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<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<G, H>);
 static_assert (std::is_layout_compatible_v<C1, D1>);