c++: Fix up ptr.~PTR () handling [PR96721]

The following testcase is miscompiled, because build_trivial_dtor_call
handles the case when instance is a pointer by adding a clobber to what
the pointer points to (which is desirable e.g. for delete) rather than the
pointer itself.  That is I think always desirable behavior for references,
but for pointers for the pseudo dtor case it is not.

2020-08-25  Jakub Jelinek  <jakub@redhat.com>

	PR c++/96721
	* cp-tree.h (build_trivial_dtor_call): Add bool argument defaulted
	to false.
	* call.c (build_trivial_dtor_call): Add NO_PTR_DEREF argument.  If
	instance is a pointer and NO_PTR_DEREF is true, clobber the pointer
	rather than what it points to.
	* semantics.c (finish_call_expr): Call build_trivial_dtor_call with
	true as NO_PTR_DEREF.

	* g++.dg/opt/flifetime-dse8.C: New test.

1 files changed, 6 insertions, 3 deletions

diff --git a/gcc/cp/call.c b/gcc/cp/call.c
index c62d9e2..4726e57 100644
--- a/gcc/cp/call.c
+++ b/gcc/cp/call.c
@@ -8430,10 +8430,12 @@ conv_binds_ref_to_prvalue (conversion *c)
 }
 
 /* Call the trivial destructor for INSTANCE, which can be either an lvalue of
-   class type or a pointer to class type.  */
+   class type or a pointer to class type.  If NO_PTR_DEREF is true and
+   INSTANCE has pointer type, clobber the pointer rather than what it points
+   to.  */
 
 tree
-build_trivial_dtor_call (tree instance)
+build_trivial_dtor_call (tree instance, bool no_ptr_deref)
 {
   gcc_assert (!is_dummy_object (instance));
 
@@ -8443,7 +8445,8 @@ build_trivial_dtor_call (tree instance)
       return fold_convert (void_type_node, instance);
     }
 
-  if (INDIRECT_TYPE_P (TREE_TYPE (instance)))
+  if (INDIRECT_TYPE_P (TREE_TYPE (instance))
+      && (!no_ptr_deref || TYPE_REF_P (TREE_TYPE (instance))))
     {
       if (VOID_TYPE_P (TREE_TYPE (TREE_TYPE (instance))))
 	goto no_clobber;