[gcc r11-9710] c++: constexpr array reference and value-initialization [PR101371]

[gcc r11-9710] c++: constexpr array reference and value-initialization [PR101371]

[Marek Polacek]

https://gcc.gnu.org/g:da47a84e277c7fbeebbf9c1e7dc1e8ba3277fe53

commit r11-9710-gda47a84e277c7fbeebbf9c1e7dc1e8ba3277fe53
Author: Marek Polacek <polacek@redhat.com>
Date:   Tue Jul 13 17:16:54 2021 -0400

    c++: constexpr array reference and value-initialization [PR101371]
    
    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.
    
    (cherry picked from commit a42f8120442cf3ba25d621bed857b5be19019d0c)

Diff:
---
 gcc/cp/constexpr.c                              | 15 +++++++++----
 gcc/testsuite/g++.dg/cpp1y/constexpr-101371-2.C | 23 ++++++++++++++++++++
 gcc/testsuite/g++.dg/cpp1y/constexpr-101371.C   | 29 +++++++++++++++++++++++++
 3 files changed, 63 insertions(+), 4 deletions(-)

diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index 4d86442ca11..f528bfd38ab 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -3859,16 +3859,23 @@ 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();
+}