From 7ece3bd8088983289731450826c238eb2bdd2db5 Mon Sep 17 00:00:00 2001
From: Jason Merrill <jason@redhat.com>
Date: Wed, 3 Jun 2020 23:50:50 -0400
Subject: [PATCH] c++: Fix complex constexpr virtual cases [PR93310].

The code in constexpr for looking up the actual type of the object and then
getting the virtual function from there broke for both of these tests: for
16, it assumed incorrectly that the DECL_VINDEX would apply to the most
derived type's vtable; for 17, it failed to consider that during
construction the base subobject is treated as being of the base type.

Fixed by just doing constant evaluation of the expression that looks up the
function in the vtable.  This means that a virtual call will involve loading
the vptr, so we will reject some calls through non-constexpr variables that
we previously accepted, but this seems appropriate to me.  None of our
testcases were affected.

gcc/cp/ChangeLog:

	PR c++/93310
	* constexpr.c (cxx_eval_constant_expression) [OBJ_TYPE_REF]:
	Evaluate OBJ_TYPE_REF_EXPR.

gcc/testsuite/ChangeLog:

	PR c++/93310
	* g++.dg/cpp2a/constexpr-virtual16.C: New test.
	* g++.dg/cpp2a/constexpr-virtual17.C: New test.
	* g++.dg/cpp2a/constexpr-new12.C: Adjust diagnostic.
---
 gcc/cp/constexpr.c                            | 44 ++-----------------
 gcc/testsuite/g++.dg/cpp2a/constexpr-new12.C  |  2 +-
 .../g++.dg/cpp2a/constexpr-virtual16.C        | 22 ++++++++++
 .../g++.dg/cpp2a/constexpr-virtual17.C        | 28 ++++++++++++
 4 files changed, 54 insertions(+), 42 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/constexpr-virtual16.C
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/constexpr-virtual17.C

diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index c7d17e9b86e..c01c42bf886 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -6373,47 +6373,9 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
       break;
 
     case OBJ_TYPE_REF:
-      {
-	/* Virtual function call.  Let the constexpr machinery figure out
-	   the dynamic type.  */
-	int token = tree_to_shwi (OBJ_TYPE_REF_TOKEN (t));
-	tree obj = OBJ_TYPE_REF_OBJECT (t);
-	obj = cxx_eval_constant_expression (ctx, obj, lval, non_constant_p,
-					    overflow_p);
-	STRIP_NOPS (obj);
-	/* We expect something in the form of &x.D.2103.D.2094; get x. */
-	if (TREE_CODE (obj) != ADDR_EXPR
-	    || !DECL_P (get_base_address (TREE_OPERAND (obj, 0))))
-	  {
-	    if (!ctx->quiet)
-	      error_at (loc, "expression %qE is not a constant expression", t);
-	    *non_constant_p = true;
-	    return t;
-	  }
-	obj = TREE_OPERAND (obj, 0);
-	while (TREE_CODE (obj) == COMPONENT_REF
-	       && DECL_FIELD_IS_BASE (TREE_OPERAND (obj, 1)))
-	  obj = TREE_OPERAND (obj, 0);
-	tree objtype = TREE_TYPE (obj);
-	if (VAR_P (obj)
-	    && DECL_NAME (obj) == heap_identifier
-	    && TREE_CODE (objtype) == ARRAY_TYPE)
-	  objtype = TREE_TYPE (objtype);
-	if (!CLASS_TYPE_P (objtype))
-	  {
-	    if (!ctx->quiet)
-	      error_at (loc, "expression %qE is not a constant expression", t);
-	    *non_constant_p = true;
-	    return t;
-	  }
-	/* Find the function decl in the virtual functions list.  TOKEN is
-	   the DECL_VINDEX that says which function we're looking for.  */
-	tree virtuals = BINFO_VIRTUALS (TYPE_BINFO (objtype));
-	if (TARGET_VTABLE_USES_DESCRIPTORS)
-	  token /= MAX (TARGET_VTABLE_USES_DESCRIPTORS, 1);
-	r = TREE_VALUE (chain_index (token, virtuals));
-	break;
-      }
+      /* Virtual function lookup.  We don't need to do anything fancy.  */
+      return cxx_eval_constant_expression (ctx, OBJ_TYPE_REF_EXPR (t),
+					   lval, non_constant_p, overflow_p);
 
     case PLACEHOLDER_EXPR:
       /* Use of the value or address of the current object.  */
diff --git a/gcc/testsuite/g++.dg/cpp2a/constexpr-new12.C b/gcc/testsuite/g++.dg/cpp2a/constexpr-new12.C
index fcf398f000e..5a3d06a5fab 100644
--- a/gcc/testsuite/g++.dg/cpp2a/constexpr-new12.C
+++ b/gcc/testsuite/g++.dg/cpp2a/constexpr-new12.C
@@ -24,4 +24,4 @@ foo ()
   return r;
 }
 
-constexpr auto a = foo ();	// { dg-error "is not a constant expression" }
+constexpr auto a = foo ();	// { dg-error "constant expression" }
diff --git a/gcc/testsuite/g++.dg/cpp2a/constexpr-virtual16.C b/gcc/testsuite/g++.dg/cpp2a/constexpr-virtual16.C
new file mode 100644
index 00000000000..8cca8a61723
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/constexpr-virtual16.C
@@ -0,0 +1,22 @@
+// Test constexpr virtual in non-primary vtable.
+// { dg-do compile { target c++20 } }
+
+struct A
+{
+  virtual constexpr int f() const { return 1; };
+};
+
+struct B
+{
+  virtual constexpr int g() const { return 2; };
+};
+
+struct C: A, B
+{
+};
+
+constexpr C c;
+
+constexpr int g(const B& b) { return b.g(); }
+static_assert (g(c) == 2);
+
diff --git a/gcc/testsuite/g++.dg/cpp2a/constexpr-virtual17.C b/gcc/testsuite/g++.dg/cpp2a/constexpr-virtual17.C
new file mode 100644
index 00000000000..a8a89952626
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/constexpr-virtual17.C
@@ -0,0 +1,28 @@
+// PR c++/93310
+// { dg-do compile { target c++20 } }
+
+struct A
+{
+  virtual constexpr char f () const
+  { return 'A'; }
+};
+
+struct B : A
+{
+  char x;
+
+  constexpr B () : x (0)
+  { x = ((A *)this)->f(); }
+
+  virtual constexpr char f () const
+  { return 'B'; }
+};
+
+struct C : B
+{
+  virtual constexpr char f () const
+  { return 'C'; }
+};
+
+constexpr C c;
+static_assert (c.x == 'B');
-- 
2.30.2