On Wed, Jul 14, 2021 at 12:15:48AM -0400, Jason Merrill wrote:
> On 7/13/21 8:15 PM, Marek Polacek wrote:
> > This PR gave me a hard time: I saw multiple issues starting with
> > different revisions. But ultimately the root cause seems to be
> > the following, and the attached patch fixes all issues I've found
> > here.
> >
> > In cxx_eval_array_reference we create a new constexpr context for the
> > CP_AGGREGATE_TYPE_P case, but we also have to create it for the
> > non-aggregate case.
>
> But not for the scalar case, surely? Other similar places check
> AGGREGATE_TYPE_P || VECTOR_TYPE_P, or !SCALAR_TYPE_P.
Yea, I suppose I should avoid doing any extra work for scalars.
> > In this test, we are evaluating
> >
> > ((B *)this)->a = rhs->a
> >
> > which means that we set ctx.object to ((B *)this)->a. Then we proceed
> > to evaluate the initializer, rhs->a. For *rhs, we eval rhs, a PARM_DECL,
> > for which we have (const B &) &c.arr[0] in the hash table. Then
> > cxx_fold_indirect_ref gives us c.arr[0]. c is evaluated to {.arr={}} so
> > c.arr is {}. Now we want c.arr[0], so we end up in cxx_eval_array_reference
> > and since we're initializing from {}, we call build_value_init which
> > gives us an AGGR_INIT_EXPR that calls 'constexpr B::B()'. Then we
> > evaluate this AGGR_INIT_EXPR and since its first argument is dummy,
> > we take ctx.object instead. But that is the wrong object, we're not
> > initializing ((B *)this)->a here. And so we wound up with an
> > initializer for A, and then crash in cxx_eval_component_reference:
> >
> > gcc_assert (DECL_CONTEXT (part) == TYPE_MAIN_VARIANT (TREE_TYPE
> > (whole)));
> >
> > where DECL_CONTEXT (part) is B (as it should be) but the type of whole
> > was A.
> >
> > With that in mind, the fix is straightforward, except that when the
> > value-init produced an AGGR_INIT_EXPR, we shouldn't set ctx.object so
> > that
> >
> > 2508 if (DECL_CONSTRUCTOR_P (fun) && !ctx->object
> > 2509 && TREE_CODE (t) == AGGR_INIT_EXPR)
> > 2510 {
> > 2511 /* We want to have an initialization target for an
> > AGGR_INIT_EXPR.
> > 2512 If we don't already have one in CTX, use the
> > AGGR_INIT_EXPR_SLOT. */
> > 2513 new_ctx.object = AGGR_INIT_EXPR_SLOT (t);
> >
> > comes into play.
>
> Hmm, setting new_ctx.object to t here looks like it should be the correct
> c.arr[0], not ((B*)this)->a. It was wrong in the current code because we
> weren't setting up new_ctx at all, but once that's fixed I don't think you
> need special AGGR_INIT_EXPR handling.
If you don't want the special AGGR_INIT_EXPR handling, we could do something
like the following. That any better?
Full testing in progress.
-- >8 --
This PR gave me a hard time: I saw multiple issues starting with
different revisions. But ultimately the root cause seems to be
the following, and the attached patch fixes all issues I've found
here.
In cxx_eval_array_reference we create a new constexpr context for the
CP_AGGREGATE_TYPE_P case, but we also have to create it for the
non-aggregate case. In this test, we are evaluating
((B *)this)->a = rhs->a
which means that we set ctx.object to ((B *)this)->a. Then we proceed
to evaluate the initializer, rhs->a. For *rhs, we eval rhs, a PARM_DECL,
for which we have (const B &) &c.arr[0] in the hash table. Then
cxx_fold_indirect_ref gives us c.arr[0]. c is evaluated to {.arr={}} so
c.arr is {}. Now we want c.arr[0], so we end up in cxx_eval_array_reference
and since we're initializing from {}, we call build_value_init which
gives us an AGGR_INIT_EXPR that calls 'constexpr B::B()'. Then we
evaluate this AGGR_INIT_EXPR and since its first argument is dummy,
we take ctx.object instead. But that is the wrong object, we're not
initializing ((B *)this)->a here. And so we wound up with an
initializer for A, and then crash in cxx_eval_component_reference:
gcc_assert (DECL_CONTEXT (part) == TYPE_MAIN_VARIANT (TREE_TYPE (whole)));
where DECL_CONTEXT (part) is B (as it should be) but the type of whole
was A.
So create a new object, if there already was one, and the element type
is not a scalar.
PR c++/101371
gcc/cp/ChangeLog:
* constexpr.c (cxx_eval_array_reference): Create a new .object
and .ctor for the non-aggregate non-scalar case too when
value-initializing.
gcc/testsuite/ChangeLog:
* g++.dg/cpp1y/constexpr-101371-2.C: New test.
* g++.dg/cpp1y/constexpr-101371.C: New test.
---
gcc/cp/constexpr.c | 15 +++++++---
.../g++.dg/cpp1y/constexpr-101371-2.C | 23 +++++++++++++++
gcc/testsuite/g++.dg/cpp1y/constexpr-101371.C | 29 +++++++++++++++++++
3 files changed, 63 insertions(+), 4 deletions(-)
create mode 100644 gcc/testsuite/g++.dg/cpp1y/constexpr-101371-2.C
create mode 100644 gcc/testsuite/g++.dg/cpp1y/constexpr-101371.C
diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index 39787f3f5d5..31fa5b66865 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -3851,16 +3851,23 @@ 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);
+ }
+ else
+ val = build_value_init (elem_type, tf_warning_or_error);
+
+ if (!SCALAR_TYPE_P (elem_type))
+ {
new_ctx = *ctx;
- new_ctx.object = t;
+ if (ctx->object)
+ /* If there was no object, don't add one: it could confuse us
+ into thinking we're modifying a const object. */
+ new_ctx.object = t;
new_ctx.ctor = build_constructor (elem_type, NULL);
ctx = &new_ctx;
}
- else
- val = build_value_init (elem_type, tf_warning_or_error);
t = cxx_eval_constant_expression (ctx, val, lval, non_constant_p,
overflow_p);
- if (CP_AGGREGATE_TYPE_P (elem_type) && t != ctx->ctor)
+ if (!SCALAR_TYPE_P (elem_type) && t != ctx->ctor)
free_constructor (ctx->ctor);
return t;
}
diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-101371-2.C
b/gcc/testsuite/g++.dg/cpp1y/constexpr-101371-2.C
new file mode 100644
index 00000000000..fb67b67c265
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-101371-2.C
@@ -0,0 +1,23 @@
+// PR c++/101371
+// { dg-do compile { target c++14 } }
+
+struct A {
+ int i;
+};
+struct B {
+ A a{};
+ constexpr B() : a() {}
+ constexpr B(const B &rhs) : a(rhs.a) {}
+};
+struct C {
+ B arr[1];
+};
+
+constexpr C
+fn ()
+{
+ C c{};
+ return c;
+}
+
+constexpr C c = fn();
diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-101371.C
b/gcc/testsuite/g++.dg/cpp1y/constexpr-101371.C
new file mode 100644
index 00000000000..b6351b806b9
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-101371.C
@@ -0,0 +1,29 @@
+// PR c++/101371
+// { dg-do compile { target c++14 } }
+
+struct A {
+ int i;
+};
+struct B {
+ A a{};
+ constexpr B() : a() {}
+ constexpr B(const B &rhs) : a(rhs.a) {}
+};
+struct C {
+ B arr[1];
+};
+
+struct X {
+ constexpr C fn () const
+ {
+ C c{};
+ return c;
+ }
+};
+
+void
+g ()
+{
+ X x;
+ constexpr auto z = x.fn();
+}
base-commit: f9c2ce1dae270d8d5dc261a57a21f96a1da5ea2d
--
2.31.1
