From 94ff4c9dd98f39280fba22d1ad0958fb25a5363b Mon Sep 17 00:00:00 2001
From: Jason Merrill <jason@redhat.com>
Date: Fri, 22 Jan 2021 13:17:10 -0500
Subject: [PATCH] c++: [[no_unique_address]] in empty base [PR98463]

In this testcase, cxx_eval_store_expression got confused trying to build up
CONSTRUCTORs for initializing a subobject because the subobject is a member
of an empty base.  In C++14 mode and below we don't build FIELD_DECLs for
empty bases, so the CONSTRUCTOR skipped the empty base, and treated the
member as a member of the derived class, which breaks.

Fixed by recognizing this situation and giving up on trying to build a
CONSTRUCTOR for the inner target at that point; since it doesn't have any
data, we don't need to actually store anything.

gcc/cp/ChangeLog:

	PR c++/98463
	* constexpr.c (get_or_insert_ctor_field): Add check.
	(cxx_eval_store_expression): Handle discontinuity of refs.

gcc/testsuite/ChangeLog:

	PR c++/98463
	* g++.dg/cpp2a/no_unique_address8.C: New test.
---
 gcc/cp/constexpr.c                             | 18 ++++++++++++++++--
 .../g++.dg/cpp2a/no_unique_address8.C          | 11 +++++++++++
 2 files changed, 27 insertions(+), 2 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/no_unique_address8.C

diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index b787919942b..c1217535761 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -3474,7 +3474,9 @@ get_or_insert_ctor_field (tree ctor, tree index, int pos_hint = -1)
     }
   else
     {
-      gcc_assert (TREE_CODE (index) == FIELD_DECL);
+      gcc_assert (TREE_CODE (index) == FIELD_DECL
+		  && (same_type_ignoring_top_level_qualifiers_p
+		      (DECL_CONTEXT (index), TREE_TYPE (ctor))));
 
       /* We must keep the CONSTRUCTOR's ELTS in FIELD order.
 	 Usually we meet initializers in that order, but it is
@@ -5277,7 +5279,7 @@ cxx_eval_store_expression (const constexpr_ctx *ctx, tree t,
 		};
 	      CONSTRUCTOR_ELTS (ary_ctor)->quick_push (elt);
 	    }
-	  
+
 	  *valp = ary_ctor;
 	}
 
@@ -5289,6 +5291,18 @@ cxx_eval_store_expression (const constexpr_ctx *ctx, tree t,
       type = refs->pop();
       tree index = refs->pop();
 
+      if (TREE_CODE (index) == FIELD_DECL
+	  && !(same_type_ignoring_top_level_qualifiers_p
+	       (DECL_CONTEXT (index), TREE_TYPE (*valp))))
+	{
+	  /* INDEX isn't a member of *valp.  This can happen if it's a member
+	     of an empty base which isn't represented with a FIELD_DECL.  Stop
+	     trying to build a CONSTRUCTOR for the inner target; we'll notice
+	     this disconnect again below and just return init.  */
+	  gcc_assert (is_empty_class (DECL_CONTEXT (index)));
+	  break;
+	}
+
       if (code == UNION_TYPE && CONSTRUCTOR_NELTS (*valp)
 	  && CONSTRUCTOR_ELT (*valp, 0)->index != index)
 	{
diff --git a/gcc/testsuite/g++.dg/cpp2a/no_unique_address8.C b/gcc/testsuite/g++.dg/cpp2a/no_unique_address8.C
new file mode 100644
index 00000000000..8b63ca37bab
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/no_unique_address8.C
@@ -0,0 +1,11 @@
+// PR c++/98463
+// { dg-do compile { target c++11 } }
+
+template <typename T> struct A { constexpr A () : a() {} [[no_unique_address]] T a; };
+template <unsigned long, typename...> struct B;
+template <unsigned long T, typename U, typename... V>
+struct B<T, U, V...> : B<1, V...>, A<U> {};
+template <unsigned long T, typename U> struct B<T, U> : A<U> {};
+template <typename... h> struct C : B<0, h...> {};
+struct D {};
+struct E { C<int, D> k; virtual ~E (); } a;
-- 
2.30.2