[PATCH v2] c++: wrong error with constexpr array and value-init [PR108158]

[Marek Polacek <polacek@redhat.com>]

On Thu, Feb 02, 2023 at 05:29:44PM -0500, Jason Merrill wrote:
> On 1/30/23 21:35, Marek Polacek wrote:
> > In this test case, we find ourselves evaluating 't' which is
> > ((const struct carray *) this)->data_[VIEW_CONVERT_EXPR<long int>(index)]
> > in cxx_eval_array_reference.  ctx->object is non-null, a RESULT_DECL, so
> > we replace it with 't':
> > 
> >    new_ctx.object = t; // result_decl replaced
> > 
> > and then we go to cxx_eval_constant_expression to evaluate an
> > AGGR_INIT_EXPR, where we end up evaluating an INIT_EXPR (which is in the
> > body of the constructor for seed_or_index):
> > 
> >    ((struct seed_or_index *) this)->value_ = NON_LVALUE_EXPR <0>
> > 
> > whereupon in cxx_eval_store_expression we go to the probe loop
> > where the 'this' is evaluated to
> > 
> >    ze_set.tables_.first_table_.data_[0]
> > 
> > so the 'object' is ze_set, but that isn't in ctx->global->get_value_ptr
> > so we fail with a bogus error.  ze_set is not there because it comes
> > from a different constexpr context (it's not in cv_cache either).
> > 
> > The problem started with r12-2304 where I added the new_ctx.object
> > replacement.  That was to prevent a type mismatch: the type of 't'
> > and ctx.object were different.
> > 
> > It seems clear that we shouldn't have replaced ctx.object here.
> > The cxx_eval_array_reference I mentioned earlier is called from
> > cxx_eval_store_expression:
> >   6257       init = cxx_eval_constant_expression (&new_ctx, init, vc_prvalue,
> >   6258                                            non_constant_p, overflow_p);
> > which already created a new context, whose .object we should be
> > using unless, for instance, INIT contained a.b and we're evaluating
> > the 'a' part, which I think was the case for r12-2304; in that case
> > ctx.object has to be something different.
> > 
> > A relatively safe fix should be to check the types before replacing
> > ctx.object, as in the below.
> 
> Agreed.  I'm trying to understand when the replacement could ever make
> sense, since 't' is not the target, it's the initializer.  The replacement
> comes from Patrick's fix for 98295, but that testcase no longer hits that
> code (likely due to changes in empty class handling).
> 
> If you add a gcc_checking_assert (false) to the replacement, does anything
> trip it?

It would trip in constexpr-101371.C, added in r12-2304.  BUT, and I would
have sworn that it ICEd when I tried, it's not necessary anymore.  So it
looks like we can simply remove the new_ctx.object line.  At least for
trunk, maybe 12 too.

Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?

-- >8 --
In this test case, we find ourselves evaluating 't' which is
((const struct carray *) this)->data_[VIEW_CONVERT_EXPR<long int>(index)]
in cxx_eval_array_reference.  ctx->object is non-null, a RESULT_DECL, so
we replace it with 't':

  new_ctx.object = t; // result_decl replaced

and then we go to cxx_eval_constant_expression to evaluate an
AGGR_INIT_EXPR, where we end up evaluating an INIT_EXPR (which is in the
body of the constructor for seed_or_index):

  ((struct seed_or_index *) this)->value_ = NON_LVALUE_EXPR <0>

whereupon in cxx_eval_store_expression we go to the probe loop
where the 'this' is evaluated to

  ze_set.tables_.first_table_.data_[0]

so the 'object' is ze_set, but that isn't in ctx->global->get_value_ptr
so we fail with a bogus error.  ze_set is not there because it comes
from a different constexpr context (it's not in cv_cache either).

The problem started with r12-2304 where I added the new_ctx.object
replacement.  That was to prevent a type mismatch: the type of 't'
and ctx.object were different.

It seems clear that we shouldn't have replaced ctx.object here.
The cxx_eval_array_reference I mentioned earlier is called from
cxx_eval_store_expression:
 6257       init = cxx_eval_constant_expression (&new_ctx, init, vc_prvalue,
 6258                                            non_constant_p, overflow_p);
which already created a new context, whose .object we should be
using unless, for instance, INIT contained a.b and we're evaluating
the 'a' part, which I think was the case for r12-2304; in that case
ctx.object has to be something different.

It no longer seems necessary to replace new_ctx.object (likely due to
changes in empty class handling).

	PR c++/108158

gcc/cp/ChangeLog:

	* constexpr.cc (cxx_eval_array_reference): Don't replace
	new_ctx.object.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1y/constexpr-108158.C: New test.
---
 gcc/cp/constexpr.cc                           |  4 ---
 gcc/testsuite/g++.dg/cpp1y/constexpr-108158.C | 32 +++++++++++++++++++
 2 files changed, 32 insertions(+), 4 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp1y/constexpr-108158.C

diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index 5b31f9c27d1..564766c8a00 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -4301,10 +4301,6 @@ cxx_eval_array_reference (const constexpr_ctx *ctx, tree t,
   if (!SCALAR_TYPE_P (elem_type))
     {
       new_ctx = *ctx;
-      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;
     }
diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-108158.C b/gcc/testsuite/g++.dg/cpp1y/constexpr-108158.C
new file mode 100644
index 00000000000..e5f5e9954e5
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-108158.C
@@ -0,0 +1,32 @@
+// PR c++/108158
+// { dg-do compile { target c++14 } }
+
+template <class T, int N> struct carray {
+  T data_[N]{};
+  constexpr T operator[](long index) const { return data_[index]; }
+};
+struct seed_or_index {
+private:
+  long value_ = 0;
+};
+template <int M> struct pmh_tables {
+  carray<seed_or_index, M> first_table_;
+  template <typename KeyType, typename HasherType>
+  constexpr void lookup(KeyType, HasherType) const {
+    first_table_[0];
+  }
+};
+template <int N> struct unordered_set {
+  int equal_;
+  carray<int, N> keys_;
+  pmh_tables<N> tables_;
+  constexpr unordered_set() : equal_{} {}
+  template <class KeyType, class Hasher>
+  constexpr auto lookup(KeyType key, Hasher hash) const {
+    tables_.lookup(key, hash);
+    return keys_;
+  }
+};
+constexpr unordered_set<3> ze_set;
+constexpr auto nocount = ze_set.lookup(4, int());
+constexpr auto nocount2 = unordered_set<3>{}.lookup(4, int());

base-commit: c9aef107ce697f58a34734d82f8d2514405c9be0
-- 
2.39.1