c++: constexpr clobber of const [PR121068]

Since r16-3022, 20_util/variant/102912.cc was failing in C++20 and above due
to wrong errors about destruction modifying a const object; destruction is
OK.

	PR c++/121068

gcc/cp/ChangeLog:

	* constexpr.cc (cxx_eval_store_expression): Allow clobber of a const
	object.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/constexpr-dtor18.C: New test.

2 files changed, 29 insertions, 0 deletions

diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index 223240a..e5438b2 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -7659,6 +7659,9 @@ cxx_eval_store_expression (const constexpr_ctx *ctx, tree t,
 	  ctx->global->destroy_value (object);
 	  return void_node;
 	}
+
+      /* Ending the lifetime of a const object is OK.  */
+      const_object_being_modified = NULL_TREE;
     }
 
   type = TREE_TYPE (object);
diff --git a/gcc/testsuite/g++.dg/cpp2a/constexpr-dtor18.C b/gcc/testsuite/g++.dg/cpp2a/constexpr-dtor18.C
new file mode 100644
index 0000000..dc0d174
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/constexpr-dtor18.C
@@ -0,0 +1,26 @@
+// PR c++/121068
+// { dg-do compile { target c++20 } }
+
+template <class T>
+constexpr void
+destroy_at (T* p) { p->~T(); }
+
+template <class T>
+struct V {
+  union {
+    unsigned char buf[sizeof (T)];
+    const T ct;
+  };
+  bool active;
+  constexpr V(): active (false) {}
+  constexpr V(T t): ct (t), active (true) { }
+  constexpr ~V() { if (active) destroy_at (&ct); }
+};
+
+constexpr char f()
+{
+  const V<int> vi {42};
+  return vi.ct;
+}
+
+static_assert (f() == 42);