[pushed] c++: empty base constexpr -fno-elide-ctors [PR105245]

[pushed] c++: empty base constexpr -fno-elide-ctors [PR105245]

[Jason Merrill]

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.

Tested x86_64-pc-linux-gnu, applying to trunk.

	PR c++/105245
	PR c++/100111

gcc/cp/ChangeLog:

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

diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index 1ce1842430d..b170053e8e1 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -5933,6 +5933,12 @@ 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
 {

base-commit: 31350635bfd90beea79b0e9220008da12bbb5d22
-- 
2.27.0