Update the C FE routine "add_flexible_array_elts_to_size" C++ FE routine "layout_var_decl" to handle the cases when the DECL is union.

PR c/53548

Add testing cases to test the _bos for flexible array members in unions
or alone in structures.

gcc/c/ChangeLog:

	PR c/53548
	* c-decl.cc (add_flexible_array_elts_to_size): Handle the cases
	when the DECL is union.

gcc/cp/ChangeLog:

	PR c/53548
	* decl.cc (layout_var_decl): Handle the cases when the DECL is
	union with a flexible array member initializer.

gcc/testsuite/ChangeLog:

	PR c/53548
	* c-c++-common/fam-in-union-alone-in-struct-bos-1.c: New test.
	* c-c++-common/fam-in-union-alone-in-struct-bos.c: New test.

1 files changed, 23 insertions, 6 deletions

diff --git a/gcc/c/c-decl.cc b/gcc/c/c-decl.cc
index 9ef2ab2..b691b91 100644
--- a/gcc/c/c-decl.cc
+++ b/gcc/c/c-decl.cc
@@ -5339,8 +5339,9 @@ zero_length_array_type_p (const_tree type)
 }
 
 /* INIT is a constructor that forms DECL's initializer.  If the final
-   element initializes a flexible array field, add the size of that
-   initializer to DECL's size.  */
+   element initializes a flexible array field, adjust the size of the
+   DECL with the initializer based on whether the DECL is a union or
+   a structure.  */
 
 static void
 add_flexible_array_elts_to_size (tree decl, tree init)
@@ -5355,10 +5356,26 @@ add_flexible_array_elts_to_size (tree decl, tree init)
   if (flexible_array_member_type_p (type))
     {
       complete_array_type (&type, elt, false);
-      DECL_SIZE (decl)
-	= size_binop (PLUS_EXPR, DECL_SIZE (decl), TYPE_SIZE (type));
-      DECL_SIZE_UNIT (decl)
-	= size_binop (PLUS_EXPR, DECL_SIZE_UNIT (decl), TYPE_SIZE_UNIT (type));
+      /* For a structure, add the size of the initializer to the DECL's
+	 size.  */
+      if (TREE_CODE (TREE_TYPE (decl)) == RECORD_TYPE)
+	{
+	  DECL_SIZE (decl)
+	    = size_binop (PLUS_EXPR, DECL_SIZE (decl), TYPE_SIZE (type));
+	  DECL_SIZE_UNIT (decl)
+	    = size_binop (PLUS_EXPR, DECL_SIZE_UNIT (decl),
+			  TYPE_SIZE_UNIT (type));
+	}
+      /* For a union, the DECL's size is the maximum of the current size
+	 and the size of the initializer.  */
+      else
+	{
+	  DECL_SIZE (decl)
+	    = size_binop (MAX_EXPR, DECL_SIZE (decl), TYPE_SIZE (type));
+	  DECL_SIZE_UNIT (decl)
+	    = size_binop (MAX_EXPR, DECL_SIZE_UNIT (decl),
+			  TYPE_SIZE_UNIT (type));
+	}
     }
 }