[gcc r11-9984] c++: empty base constexpr -fno-elide-ctors [PR105245]

[gcc r11-9984] c++: empty base constexpr -fno-elide-ctors [PR105245]

[Jason Merrill]

https://gcc.gnu.org/g:fe81f5bd3c3e764d1355eda3e44e37cec99cd23c

commit r11-9984-gfe81f5bd3c3e764d1355eda3e44e37cec99cd23c
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 d72a8712dab..57a45e602cc 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -5578,6 +5578,12 @@ 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, nullptr);
+	  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
 {