DR 1813 PR c++/83374 - __is_standard_layout wrong for a class with repeated bases.

	DR 1813
	PR c++/83374 - __is_standard_layout wrong for a class with repeated bases.
	* class.c (check_bases): Set CLASSTYPE_NON_STD_LAYOUT for a class if
	CLASSTYPE_REPEATED_BASE_P is true.

	* g++.dg/ext/is_std_layout3.C: New test.
	* g++.dg/ext/is_std_layout4.C: New test.

From-SVN: r273139

5 files changed, 48 insertions, 1 deletions

diff --git a/gcc/cp/ChangeLog b/gcc/cp/ChangeLog
index c1dfb7c..f24a096 100644
--- a/gcc/cp/ChangeLog
+++ b/gcc/cp/ChangeLog
@@ -1,3 +1,10 @@
+2019-07-04  Marek Polacek  <polacek@redhat.com>
+
+	DR 1813
+	PR c++/83374 - __is_standard_layout wrong for a class with repeated bases.
+	* class.c (check_bases): Set CLASSTYPE_NON_STD_LAYOUT for a class if
+	CLASSTYPE_REPEATED_BASE_P is true.
+
 2019-07-04  Andrew Stubbs  <ams@codesourcery.com>
 
 	* cp-tree.h (cp_omp_emit_unmappable_type_notes): New prototype.
diff --git a/gcc/cp/class.c b/gcc/cp/class.c
index 73291b3..f77b7f4 100644
--- a/gcc/cp/class.c
+++ b/gcc/cp/class.c
@@ -1715,11 +1715,15 @@ check_bases (tree t,
 	      && (same_type_ignoring_top_level_qualifiers_p
 		  (TREE_TYPE (field), basetype)))
 	    CLASSTYPE_NON_STD_LAYOUT (t) = 1;
+	  /* DR 1813:
+	     ...has at most one base class subobject of any given type...  */
+	  else if (CLASSTYPE_REPEATED_BASE_P (t))
+	    CLASSTYPE_NON_STD_LAYOUT (t) = 1;
 	  else
 	    /* ...either has no non-static data members in the most-derived
 	       class and at most one base class with non-static data
 	       members, or has no base classes with non-static data
-	       members */
+	       members.  FIXME This was reworded in DR 1813.  */
 	    for (basefield = TYPE_FIELDS (basetype); basefield;
 		 basefield = DECL_CHAIN (basefield))
 	      if (TREE_CODE (basefield) == FIELD_DECL
diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog
index a57c230..98ad880 100644
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -1,3 +1,10 @@
+2019-07-04  Marek Polacek  <polacek@redhat.com>
+
+	DR 1813
+	PR c++/83374 - __is_standard_layout wrong for a class with repeated bases.
+	* g++.dg/ext/is_std_layout3.C: New test.
+	* g++.dg/ext/is_std_layout4.C: New test.
+
 2019-07-05  Richard Biener  <rguenther@suse.de>
 
 	* gcc.dg/tree-ssa/ssa-fre-77.c: New testcase.
diff --git a/gcc/testsuite/g++.dg/ext/is_std_layout3.C b/gcc/testsuite/g++.dg/ext/is_std_layout3.C
new file mode 100644
index 0000000..b0555c8
--- /dev/null
+++ b/gcc/testsuite/g++.dg/ext/is_std_layout3.C
@@ -0,0 +1,18 @@
+// DR 1813
+// PR c++/83374 - __is_standard_layout wrong for a class with repeated bases.
+// { dg-do compile { target c++11 } }
+
+struct B { int i; };        // standard-layout class
+struct C : B { };           // standard-layout class
+struct D : C { };           // standard-layout class
+struct E : D { char : 4; }; // not a standard-layout class
+static_assert( __is_standard_layout(B), "" );
+static_assert( __is_standard_layout(C), "" );
+static_assert( __is_standard_layout(D), "" );
+static_assert( ! __is_standard_layout(E), "" );
+
+struct Q {};
+struct S : Q { };
+struct T : Q { };
+struct U : S, T { };         // not a standard-layout class
+static_assert( ! __is_standard_layout(U), "" );
diff --git a/gcc/testsuite/g++.dg/ext/is_std_layout4.C b/gcc/testsuite/g++.dg/ext/is_std_layout4.C
new file mode 100644
index 0000000..09c0098
--- /dev/null
+++ b/gcc/testsuite/g++.dg/ext/is_std_layout4.C
@@ -0,0 +1,11 @@
+// DR 1813
+// PR c++/83374 - __is_standard_layout wrong for a class with repeated bases.
+// { dg-do compile { target c++11 } }
+
+struct R { };
+struct Q { };
+struct S : R { };
+struct T : Q { };
+struct U : S, T { };
+// No repeated base class subobjects.
+static_assert(__is_standard_layout(U), "");