[gcc r10-10716] c++: cxx_eval_array_reference and empty elem type [PR101194]

[Patrick Palka <ppalka@gcc.gnu.org>]

https://gcc.gnu.org/g:5fcedef529c7564b6485ab2893c08865798a66ec

commit r10-10716-g5fcedef529c7564b6485ab2893c08865798a66ec
Author: Patrick Palka <ppalka@redhat.com>
Date:   Wed Jun 30 20:44:52 2021 -0400

    c++: cxx_eval_array_reference and empty elem type [PR101194]
    
    Here the initializer for x is represented as an empty CONSTRUCTOR due to
    its empty element type.  So during constexpr evaluation of the ARRAY_REF
    x[0], we end up trying to value initialize the omitted element at index 0,
    which fails because the element type is not default constructible.
    
    This patch makes cxx_eval_array_reference specifically handle the case
    where the element type is an empty type.
    
            PR c++/101194
    
    gcc/cp/ChangeLog:
    
            * constexpr.c (cxx_eval_array_reference): When the element type
            is an empty type and the corresponding element is omitted, just
            return an empty CONSTRUCTOR instead of attempting value
            initialization.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/cpp0x/constexpr-empty16.C: New test.
    
    (cherry picked from commit a688c284dd3848b6c4ea553035f0f9769fb4fbc9)

Diff:
---
 gcc/cp/constexpr.c                             |  4 +++-
 gcc/testsuite/g++.dg/cpp0x/constexpr-empty16.C | 10 ++++++++++
 2 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index dd0e955ed7b..28b0a88a658 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -3637,7 +3637,9 @@ cxx_eval_array_reference (const constexpr_ctx *ctx, tree t,
      directly for non-aggregates to avoid creating a garbage CONSTRUCTOR.  */
   tree val;
   constexpr_ctx new_ctx;
-  if (CP_AGGREGATE_TYPE_P (elem_type))
+  if (is_really_empty_class (elem_type, /*ignore_vptr*/false))
+    return build_constructor (elem_type, NULL);
+  else if (CP_AGGREGATE_TYPE_P (elem_type))
     {
       tree empty_ctor = build_constructor (init_list_type_node, NULL);
       val = digest_init (elem_type, empty_ctor, tf_warning_or_error);
diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-empty16.C b/gcc/testsuite/g++.dg/cpp0x/constexpr-empty16.C
new file mode 100644
index 00000000000..79be244a1d0
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-empty16.C
@@ -0,0 +1,10 @@
+// PR c++/101194
+// { dg-do compile { target c++11 } }
+
+struct nodefault {
+  constexpr nodefault(int) { }
+};
+
+constexpr nodefault x[1] = { nodefault{0} };
+
+constexpr nodefault y = x[0];