[gcc r10-10722] c++: empty base constexpr -fno-elide-ctors [PR105245]

[gcc r10-10722] c++: empty base constexpr -fno-elide-ctors [PR105245]

[Jason Merrill]

https://gcc.gnu.org/g:6c7905a9f10d28dfd27ddf21d3bf38a3e261ee10

commit r10-10722-g6c7905a9f10d28dfd27ddf21d3bf38a3e261ee10
Author: Jason Merrill <jason@redhat.com>
Date:   Tue Apr 12 17:46:59 2022 -0400

    c++: empty base constexpr -fno-elide-ctors [PR105245]
    
    The patch for 100111 extended our handling of empty base elision to the case
    where the derived class has no other fields, but we still need to make sure
    that there's some initializer for the derived object.
    
            PR c++/105245
            PR c++/100111
    
    gcc/cp/ChangeLog:
    
            * constexpr.c (cxx_eval_store_expression): Build a CONSTRUCTOR
            as needed in empty base handling.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/cpp1y/constexpr-empty2.C: Add -fno-elide-constructors.

Diff:
---
 gcc/cp/constexpr.c                            | 6 ++++++
 gcc/testsuite/g++.dg/cpp1y/constexpr-empty2.C | 1 +
 2 files changed, 7 insertions(+)

diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index 28b0a88a658..31af214a28c 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -5149,6 +5149,12 @@ cxx_eval_store_expression (const constexpr_ctx *ctx, tree t,
     {
       /* See above on initialization of empty bases.  */
       gcc_assert (is_empty_class (TREE_TYPE (init)) && !lval);
+      if (!*valp)
+	{
+	  /* But do make sure we have something in *valp.  */
+	  *valp = build_constructor (type, NULL);
+	  CONSTRUCTOR_NO_CLEARING (*valp) = no_zero_init;
+	}
       return init;
     }
   else
diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-empty2.C b/gcc/testsuite/g++.dg/cpp1y/constexpr-empty2.C
index 2acfa98364b..9768b89904e 100644
--- a/gcc/testsuite/g++.dg/cpp1y/constexpr-empty2.C
+++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-empty2.C
@@ -1,4 +1,5 @@
 // { dg-do compile { target c++14 } }
+// { dg-additional-options -fno-elide-constructors }
 
 struct A
 {