On 13/04/2018 16:06, Jason Merrill wrote:
On Fri, Apr 13, 2018 at 5:05 AM, Paolo Carlini <paolo.carl...@oracle.com> wrote:

in this error-recovery regression, after a sensible error message issued by
cxx_constant_init, store_init_value stores an error_mark_node as
DECL_INITIAL of the VAR_DECL for 'j'. This error_mark_node reappears much
later, to cause a crash during gimplification. As far as I know, the choice
of storing error_mark_nodes too is the outcome of a rather delicate
error-recovery strategy and I don't think we want to revisit it at this
time, thus the remaining option is catching later the error_mark_node, at a
"good" time. I note, in passing, that the do loop in gimplify_expr which
uses the crashing STRIP_USELESS_TYPE_CONVERSION seems a bit lacking from the
error-recovery point of view, because at each iteration it *does* cover for
error_operand_p (save_expr) but only immediately after the call, when it's
too late.

All the above said, I believe that at least for the 8.1.0 needs we may want
to catch the error_mark_node in cp_build_modify_expr, when we are handling
the assignment 'a.n = j;': convert_for_assignment produces a NOP_EXPR from
the VAR_DECL for 'j' which then cp_convert_and_check regenerates (deep in
convert_to_integer_1 via maybe_fold_build1_loc) in the final bare-bone form,
with the error_mark_node as the first operand. Passes testing on
We should avoid wrapping an error_mark_node in a NOP_EXPR in the first place.
Basing on my analysis, that's easy to do, in convert_to_integer_1. I wasn't sure we wanted to touch such basic facilities ;) Implementation-wise, I wondered whether we wanted to handle NOP_EXPR and error_mark_node specially inside build1 itself, but, again, that seems a bit invasive, plus all the build* facilities always allocate a new node as the first step. Anyway, fully testing the below will require a bit of time, shall I go ahead with that, given that g++.dg passed?


Index: convert.c
--- convert.c   (revision 259376)
+++ convert.c   (working copy)
@@ -743,6 +743,8 @@ convert_to_integer_1 (tree type, tree expr, bool d
          expr = convert (lang_hooks.types.type_for_mode
                          (TYPE_MODE (type), TYPE_UNSIGNED (type)), expr);
+         if (expr == error_mark_node)
+           return error_mark_node;
          return maybe_fold_build1_loc (dofold, loc, NOP_EXPR, type, expr);
Index: testsuite/g++.dg/cpp0x/pr85112.C
--- testsuite/g++.dg/cpp0x/pr85112.C    (nonexistent)
+++ testsuite/g++.dg/cpp0x/pr85112.C    (working copy)
@@ -0,0 +1,17 @@
+// PR c++/85112
+// { dg-do compile { target c++11 } }
+struct A
+  int m;
+  int n : 4;
+int i;  // { dg-message "not const" }
+void foo()
+  constexpr int j = i;  // { dg-error "not usable" }
+  A a;
+  a.n = j;

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