c++: Fix ICE during constexpr virtual call evaluation [PR93633]

The first (valid) testcase ICEs because for
  A *a = new B ();
  a->foo (); // virtual method call
we actually see &heap  and the "heap " objects don't have the class or
whatever else type was used in new expression, but an array type containing
one (or more of those for array new) and so when using TYPE_BINFO (objtype)
on it we ICE.
This patch handles this special case, and otherwise punts (as shown e.g. in
the second testcase, where because the heap object is already deleted,
we don't really want to allow it to be used.

2020-02-09  Jakub Jelinek  <jakub@redhat.com>

	PR c++/93633
	* constexpr.c (cxx_eval_constant_expression): If obj is heap var with
	ARRAY_TYPE, use the element type.  Punt if objtype after that is not
	a class type.

	* g++.dg/cpp2a/constexpr-new11.C: New test.
	* g++.dg/cpp2a/constexpr-new12.C: New test.
	* g++.dg/cpp2a/constexpr-new13.C: New test.

6 files changed, 108 insertions, 0 deletions

diff --git a/gcc/cp/ChangeLog b/gcc/cp/ChangeLog
index dff6a17..90f26c9 100644
--- a/gcc/cp/ChangeLog
+++ b/gcc/cp/ChangeLog
@@ -1,3 +1,10 @@
+2020-02-09  Jakub Jelinek  <jakub@redhat.com>
+
+	PR c++/93633
+	* constexpr.c (cxx_eval_constant_expression): If obj is heap var with
+	ARRAY_TYPE, use the element type.  Punt if objtype after that is not
+	a class type.
+
 2020-02-08  Jason Merrill  <jason@redhat.com>
 
 	PR c++/90691
diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index 192ba56..6495cf8 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -6063,6 +6063,17 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
 	       && 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));
diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog
index c47f3fb..6680675 100644
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -1,3 +1,10 @@
+2020-02-09  Jakub Jelinek  <jakub@redhat.com>
+
+	PR c++/93633
+	* g++.dg/cpp2a/constexpr-new11.C: New test.
+	* g++.dg/cpp2a/constexpr-new12.C: New test.
+	* g++.dg/cpp2a/constexpr-new13.C: New test.
+
 2020-02-08  Andrew Pinski  <apinski@marvel.com>
 
 	PR target/91927
diff --git a/gcc/testsuite/g++.dg/cpp2a/constexpr-new11.C b/gcc/testsuite/g++.dg/cpp2a/constexpr-new11.C
new file mode 100644
index 0000000..26658d0
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/constexpr-new11.C
@@ -0,0 +1,31 @@
+// PR c++/93633
+// { dg-do compile { target c++2a } }
+
+struct A {
+  constexpr A () : a (0) {}
+  constexpr virtual int foo () { return 1 + a * 4; }
+  int a;
+};
+
+struct B : A {
+  constexpr B () : b (0) {}
+  constexpr virtual int foo () { return 0 + b * 4; }
+  int b;
+};
+
+constexpr int
+foo ()
+{
+  A *a = new B ();
+  a->a = 4;
+  int r = a->foo ();
+  delete a;
+  return r;
+}
+
+int
+main ()
+{
+  constexpr auto a = foo ();
+  static_assert (a == 0);
+}
diff --git a/gcc/testsuite/g++.dg/cpp2a/constexpr-new12.C b/gcc/testsuite/g++.dg/cpp2a/constexpr-new12.C
new file mode 100644
index 0000000..2dedcd2
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/constexpr-new12.C
@@ -0,0 +1,26 @@
+// PR c++/93633
+// { dg-do compile { target c++2a } }
+
+struct A {
+  constexpr A () : a (0) {}
+  constexpr virtual int foo () { return 1 + a * 4; }
+  int a;
+};
+
+struct B : A {
+  constexpr B () : b (0) {}
+  constexpr virtual int foo () { return 0 + b * 4; }
+  int b;
+};
+
+constexpr int
+foo ()
+{
+  A *a = new B ();
+  a->a = 4;
+  delete a;
+  int r = a->foo ();
+  return r;
+}
+
+constexpr auto a = foo ();	// { dg-error "is not a constant expression" }
diff --git a/gcc/testsuite/g++.dg/cpp2a/constexpr-new13.C b/gcc/testsuite/g++.dg/cpp2a/constexpr-new13.C
new file mode 100644
index 0000000..2121903
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/constexpr-new13.C
@@ -0,0 +1,26 @@
+// PR c++/93633
+// { dg-do compile { target c++2a } }
+
+struct A {
+  constexpr A () : a (0) {}
+  virtual int foo () { return 1 + a * 4; }
+  int a;
+};
+
+struct B : A {
+  constexpr B () : b (0) {}
+  virtual int foo () { return 0 + b * 4; }	// { dg-message "declared here" }
+  int b;
+};
+
+constexpr int
+foo ()
+{
+  A *a = new B ();
+  a->a = 4;
+  int r = a->foo ();	// { dg-error "call to non-.constexpr. function" }
+  delete a;
+  return r;
+}
+
+constexpr auto a = foo ();