On Fri, 29 Jan 2021, Jason Merrill wrote:
> On 1/29/21 12:28 PM, Patrick Palka wrote:
> > In this testcase we're crash during constexpr evaluation of the
> > ARRAY_REF b[0] as part of folding the lambda's by-copy capture of b
> > (which is encoded as a VEC_INIT_EXPR<b>). Since A's default constructor
> > is not yet defined, b's initializer is not actually constant, but
> > because A is an empty type, evaluation of the referent b from
> > cxx_eval_array_ref yields an empty CONSTRUCTOR. From there we proceed
> > to {}-initialize the missing array element at index 0. We crash from
> > verify_ctor_sanity during evaluation of this initializer because we
> > updated constexpr_ctx::ctor without updating ::object; the former has
> > type A[3] and the latter is the target of the TARGET_EXPR for b[0][0]
> > created from cxx_eval_vec_init_1 (and so has type A).
> >
> > This patch conservatively fixes this issue by clearing new_ctx.object at
> > the same time that we set new_ctx.ctor. Strictly speaking, the object
> > under construction should perhaps be the ARRAY_REF itself
>
> Yes.
>
> > but I haven't
> > been able to come up with a testcase for which this difference matters.
>
> I suspect that any case where it would matter wouldn't get to this point,
> because we would have already built up the initializer under digest_init.
>
> But I also think it shouldn't hurt to use 't' as new_ctx.object, so let's do
> that.
Sounds good. I had convinced myself earlier that using 't' wouldn't be
quite right and that we ought to use something like
cxx_eval_array_reference(t, /*lval=*/true) as new_ctx.object instead in
order to ensure the operands of the ARRAY_REF are fully reduced. But it
seems this isn't necessary since the routines which inspect ctx->object
already pass it through cxx_eval_constant_expression appropriately. So
using 't', even when it's unreduced, should be safe in this respect.
Does the following look OK for trunk/10?
-- >8 --
Subject: [PATCH] c++: Fix ICE from verify_ctor_sanity [PR98295]
In this testcase we're crashing during constexpr evaluation of the
ARRAY_REF b[0] as part of evaluation of the lambda's by-copy capture of b
(which is encoded as a VEC_INIT_EXPR<b>). Since A's constexpr default
constructor is not yet defined, b's initialization is not actually
constant, but because A is an empty type, evaluation of the referent b
from cxx_eval_array_ref is successful and yields an empty CONSTRUCTOR.
Since the CONSTRUCTOR is empty, we {}-initialize the desired array
element, and we end up crashing from verify_ctor_sanity during
evaluation of this initializer because we updated new_ctx.ctor without
updating new_ctx.object: the former now has type A[3] and the latter is
still the target of a TARGET_EXPR for b[0][0] created from
cxx_eval_vec_init_1 (and so has type A).
This patch fixes this by setting new_ctx.object appropriately at the
same time that we set new_ctx.ctor from cxx_eval_array_reference.
gcc/cp/ChangeLog:
PR c++/98295
* constexpr.c (cxx_eval_array_reference): Also set
new_ctx.object when setting new_ctx.ctor.
gcc/testsuite/ChangeLog:
PR c++/98295
* g++.dg/cpp0x/constexpr-98295.C: New test.
---
gcc/cp/constexpr.c | 1 +
gcc/testsuite/g++.dg/cpp0x/constexpr-98295.C | 11 +++++++++++
2 files changed, 12 insertions(+)
create mode 100644 gcc/testsuite/g++.dg/cpp0x/constexpr-98295.C
diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index baa97a0ef17..1dbc2db9643 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -3760,6 +3760,7 @@ cxx_eval_array_reference (const constexpr_ctx *ctx, tree
t,
tree empty_ctor = build_constructor (init_list_type_node, NULL);
val = digest_init (elem_type, empty_ctor, tf_warning_or_error);
new_ctx = *ctx;
+ new_ctx.object = t;
new_ctx.ctor = build_constructor (elem_type, NULL);
ctx = &new_ctx;
}
diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-98295.C
b/gcc/testsuite/g++.dg/cpp0x/constexpr-98295.C
new file mode 100644
index 00000000000..930bd5a67da
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-98295.C
@@ -0,0 +1,11 @@
+// PR c++/98295
+// { dg-do compile { target c++11 } }
+
+struct A { constexpr A(); };
+
+void f() {
+ A b[2][3];
+ [b] { };
+}
+
+constexpr A::A() {}
--
2.30.0.335.ge6362826a0
