c++: fix arm-eabi crash building libstdc++ [PR105529]

My recent change to cxx_eval_store_expression asserts that the target and
value can only end up having different types in the case of an empty base;
this was crashing arm-eabi compilers because in that ABI [cd]tors
return *this, and weren't converting it to void* first.

This also shares the 'return this' code between the three places it occurs.

Thanks to Marek for the tests.

	PR c++/105529

gcc/cp/ChangeLog:

	* decl.cc (maybe_return_this): Replace...
	(finish_constructor_body, finish_destructor_body): ...these.
	(finish_function_body): Call it.
	* optimize.cc (build_delete_destructor_body): Call it.
	* cp-tree.h (maybe_return_this): Declare.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/constexpr-dtor13.C: New test.
	* g++.dg/cpp2a/constexpr-dtor14.C: New test.

1 files changed, 1 insertions, 8 deletions

diff --git a/gcc/cp/optimize.cc b/gcc/cp/optimize.cc
index 13ab8b7..5c134fd 100644
--- a/gcc/cp/optimize.cc
+++ b/gcc/cp/optimize.cc
@@ -163,14 +163,7 @@ build_delete_destructor_body (tree delete_dtor, tree complete_dtor)
 
   /* Return the address of the object.
      ??? How is it useful to return an invalid address?  */
-  if (targetm.cxx.cdtor_returns_this ())
-    {
-      tree val = DECL_ARGUMENTS (delete_dtor);
-      suppress_warning (val, OPT_Wuse_after_free);
-      val = build2 (MODIFY_EXPR, TREE_TYPE (val),
-                    DECL_RESULT (delete_dtor), val);
-      add_stmt (build_stmt (0, RETURN_EXPR, val));
-    }
+  maybe_return_this ();
 }
 
 /* Return name of comdat group for complete and base ctor (or dtor)