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 x86_64-linux.

Thanks, Paolo.


2018-04-13  Paolo Carlini  <paolo.carl...@oracle.com>

        PR c++/85112
        * typeck.c (cp_build_modify_expr): Return error_mark_node upon
        an error_mark_node wrapped in a NOP_EXPR too.

2018-04-13  Paolo Carlini  <paolo.carl...@oracle.com>

        PR c++/85112
        * g++.dg/cpp0x/pr85112.C: New.
Index: cp/typeck.c
--- cp/typeck.c (revision 259359)
+++ cp/typeck.c (working copy)
@@ -8234,7 +8234,9 @@ cp_build_modify_expr (location_t loc, tree lhs, en
                         TREE_OPERAND (newrhs, 0));
-  if (newrhs == error_mark_node)
+  if (newrhs == error_mark_node
+      || (TREE_CODE (newrhs) == NOP_EXPR
+         && TREE_OPERAND (newrhs, 0) == error_mark_node))
     return error_mark_node;
   if (c_dialect_objc () && flag_objc_gc)
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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