[gcc r13-5693] c++: wrong error with constexpr array and value-init [PR108158]

[Marek Polacek <mpolacek@gcc.gnu.org>]

https://gcc.gnu.org/g:27ac6a707e7438c3cec79c24f5d53de79493e2f8

commit r13-5693-g27ac6a707e7438c3cec79c24f5d53de79493e2f8
Author: Marek Polacek <polacek@redhat.com>
Date:   Mon Jan 30 09:02:00 2023 -0500

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

Diff:
---
 gcc/cp/constexpr.cc                           |  4 ----
 gcc/testsuite/g++.dg/cpp1y/constexpr-108158.C | 32 +++++++++++++++++++++++++++
 2 files changed, 32 insertions(+), 4 deletions(-)

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());