C++ PATCH for c++/80829, ICE with constexpr copy of base

[Jason Merrill]

The constexpr code uses the CONSTRUCTOR_NO_IMPLICIT_ZERO flag to track
partially-initialized aggregates, but we were failing to clear it on
base subobjects.

Tested x86_64-pc-linux-gnu, applying to trunk and 7.

commit 2e3142bcd6fde9f9ac22928718e55584a6255286
Author: Jason Merrill <ja...@redhat.com>
Date:   Mon Jun 19 15:15:14 2017 -0400

            PR c++/80829 - ICE with constexpr copy of base subobject.
    
            * constexpr.c (clear_no_implicit_zero): New.
            (cxx_eval_call_expression): Call it.

diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index 569a247..5a57452 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -1394,6 +1394,21 @@ cxx_eval_internal_function (const constexpr_ctx *ctx, 
tree t,
   return t;
 }
 
+/* Clean CONSTRUCTOR_NO_IMPLICIT_ZERO from CTOR and its sub-aggregates.  */
+
+static void
+clear_no_implicit_zero (tree ctor)
+{
+  if (CONSTRUCTOR_NO_IMPLICIT_ZERO (ctor))
+    {
+      CONSTRUCTOR_NO_IMPLICIT_ZERO (ctor) = false;
+      tree elt; unsigned HOST_WIDE_INT idx;
+      FOR_EACH_CONSTRUCTOR_VALUE (CONSTRUCTOR_ELTS (ctor), idx, elt)
+       if (TREE_CODE (elt) == CONSTRUCTOR)
+         clear_no_implicit_zero (elt);
+    }
+}
+
 /* Subroutine of cxx_eval_constant_expression.
    Evaluate the call expression tree T in the context of OLD_CALL expression
    evaluation.  */
@@ -1697,7 +1712,7 @@ cxx_eval_call_expression (const constexpr_ctx *ctx, tree 
t,
 
   /* The result of a constexpr function must be completely initialized.  */
   if (TREE_CODE (result) == CONSTRUCTOR)
-    CONSTRUCTOR_NO_IMPLICIT_ZERO (result) = false;
+    clear_no_implicit_zero (result);
 
   pop_cx_call_context ();
   return unshare_constructor (result);
diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-base5.C 
b/gcc/testsuite/g++.dg/cpp0x/constexpr-base5.C
new file mode 100644
index 0000000..84700bc
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-base5.C
@@ -0,0 +1,15 @@
+// PR c++/80829
+// { dg-do compile { target c++11 } }
+
+struct A {
+  constexpr A(int a) : _a(a) {}
+  int _a;
+};
+
+struct B : public A {
+  constexpr B(int a) : A(a) {}
+};
+
+int main() {
+  constexpr A a = B(10);
+}