PR c++/99074 - crash in dynamic_cast<>() on null pointer

libstdc++-v3/ChangeLog:

	PR c++/99074
	* libsupc++/dyncast.cc (__dynamic_cast): Return null when
	first argument is null.

gcc/testsuite/ChangeLog:

	PR c++/99074
	* g++.dg/warn/Wnonnull11.C: New test.

gcc/testsuite/g++.dg/warn/Wnonnull11.C

@@ -0,0 +1,40 @@
+/* PR c++/99074 - gcc 8 and above is crashing with dynamic_cast<>() on null
+   pointer with optimization level -O1 and above
+   { dg-do run }
+   { dg-options "-O1 -Wall" } */
+
+class Base
+{
+public:
+  virtual ~Base() {}
+  virtual void op() = 0;
+};
+
+class Object: public virtual Base { };
+
+class AbstractBase: public virtual Base
+{
+public:
+  Object* _to_object ()
+  {
+    return dynamic_cast<Object*>(this);   // { dg-warning "\\\[-Wnonnull" "" { xfail *-*-* } }
+  }
+};
+
+class MyAbstractClass: public virtual AbstractBase
+{
+public:
+  static MyAbstractClass* _nil () { return 0; }
+};
+
+
+int main ()
+{
+  MyAbstractClass *my_abs_type = MyAbstractClass::_nil ();
+  AbstractBase *abs_base = my_abs_type;
+  Object *obj = abs_base->_to_object ();
+
+  __builtin_printf ("object is: %p\n", obj);
+
+  return 0;
+}

libstdc++-v3/libsupc++/dyncast.cc

@@ -47,6 +47,9 @@ __dynamic_cast (const void *src_ptr,    // object started from
                 const __class_type_info *dst_type, // desired target type
                 ptrdiff_t src2dst) // how src and dst are related
   {
+  if (!src_ptr)
+    /* Handle precondition violations gracefully.  */
+    return NULL;
   const void *vtable = *static_cast <const void *const *> (src_ptr);
   const vtable_prefix *prefix =
     (adjust_pointer <vtable_prefix>