Add gdb::nullopt, from C++17's std::nullopt

Message ID 20200519185537.87677-1-cbiesinger@chromium.org
State Superseded
Headers show
Series
  • Add gdb::nullopt, from C++17's std::nullopt
Related show

Commit Message

Shahab Vahedi via Gdb-patches May 19, 2020, 6:55 p.m.
From: Christian Biesinger <cbiesinger@google.com>


This provides a clearer way to initialize a gdb::optional. E.g.:

  gdb::optional<foo> bar () {
    return gdb::nullopt;
  }

or

  void foo (gdb::optional<bar>) {
  }

  foo (gdb::nullopt);

Also updates all places I could find that use {} or optional() to instead
use nullopt.

gdb/ChangeLog:

2020-05-19  Christian Biesinger  <cbiesinger@google.com>

	* cli/cli-option.c (struct option_def_and_value): Update to use nullopt.
	(parse_option): Likewise.
	* dwarf2/die.h (struct die_info): Likewise.
	* dwarf2/read.c (dw2_expand_symtabs_for_function): Likewise.
	(dw2_debug_names_expand_symtabs_for_function): Likewise.
	(lookup_dwo_id): Likewise.
	* fbsd-tdep.c (fbsd_make_note_desc): Likewise.
	* inf-child.c (inf_child_target::fileio_readlink): Likewise.
	* linux-nat.c (linux_nat_target::fileio_readlink): Likewise.
	* python/python.c (gdbpy_colorize): Likewise.
	* remote.c (remote_target::fileio_readlink): Likewise.
	* solib-aix.c (solib_aix_parse_libraries): Likewise.
	* solib-svr4.c (read_program_header): Likewise.
	(read_program_headers_from_bfd): Likewise.
	* symtab.h (struct general_symbol_info): Likewise.
	* target.c (target_read_alloc_1): Likewise.
	(target_read_stralloc): Likewise.
	(target_get_osdata): Likewise.
	(target_ops::fileio_readlink): Likewise.
	(target_fileio_readlink): Likewise.
	* xml-support.c (xml_fetch_content_from_file): Likewise.
	* xml-tdesc.c (target_fetch_description_xml): Likewise.
	(string_read_description_xml): Likewise.

gdbsupport/ChangeLog:

2020-05-19  Christian Biesinger  <cbiesinger@google.com>

	* gdb_optional.h (struct nullopt_t): Add.
	(nullopt): Add.
	(class optional): Add nullopt_t constructor.

Change-Id: I371f05484a3b85cc6d54a8baead2b4f64e6ee308
---
 gdb/cli/cli-option.c      | 24 ++++++++++++------------
 gdb/dwarf2/die.h          |  2 +-
 gdb/dwarf2/read.c         |  7 ++++---
 gdb/fbsd-tdep.c           |  2 +-
 gdb/inf-child.c           |  4 ++--
 gdb/linux-nat.c           |  2 +-
 gdb/python/python.c       | 20 ++++++++++----------
 gdb/remote.c              |  4 ++--
 gdb/solib-aix.c           |  4 ++--
 gdb/solib-svr4.c          | 24 ++++++++++++------------
 gdb/symtab.h              |  3 +--
 gdb/target.c              | 10 +++++-----
 gdb/xml-support.c         |  4 ++--
 gdb/xml-tdesc.c           |  8 ++++----
 gdbsupport/gdb_optional.h | 14 ++++++++++++++
 15 files changed, 73 insertions(+), 59 deletions(-)

-- 
2.26.2.761.g0e0b3e54be-goog

Comments

Simon Marchi May 19, 2020, 7:15 p.m. | #1
On 2020-05-19 2:55 p.m., cbiesinger--- via Gdb-patches wrote:
> diff --git a/gdbsupport/gdb_optional.h b/gdbsupport/gdb_optional.h

> index 02a87f6ee4..d1e70e4be9 100644

> --- a/gdbsupport/gdb_optional.h

> +++ b/gdbsupport/gdb_optional.h

> @@ -25,12 +25,17 @@

>  namespace gdb

>  {

>  

> +struct nullopt_t {

> +    explicit constexpr nullopt_t (int) {}

> +};


Formatting of curly braces.

And as mentioned on IRC, there are probably tests in gdb/unittests/optional we can
enable for nullopt.

Simon

Patch

diff --git a/gdb/cli/cli-option.c b/gdb/cli/cli-option.c
index a24eca457d..08457e4e03 100644
--- a/gdb/cli/cli-option.c
+++ b/gdb/cli/cli-option.c
@@ -62,7 +62,7 @@  struct option_def_and_value
 
   /* Constructor.  */
   option_def_and_value (const option_def &option_, void *ctx_,
-			gdb::optional<option_value> &&value_ = {})
+			gdb::optional<option_value> &&value_ = gdb::nullopt)
     : option (option_),
       ctx (ctx_),
       value (std::move (value_))
@@ -185,15 +185,15 @@  parse_option (gdb::array_view<const option_def_group> options_group,
 	      parse_option_completion_info *completion = nullptr)
 {
   if (*args == nullptr)
-    return {};
+    return gdb::nullopt;
   else if (**args != '-')
     {
       if (have_delimiter)
 	error (_("Unrecognized option at: %s"), *args);
-      return {};
+      return gdb::nullopt;
     }
   else if (check_for_argument (args, "--"))
-    return {};
+    return gdb::nullopt;
 
   /* Skip the initial '-'.  */
   const char *arg = *args + 1;
@@ -216,7 +216,7 @@  parse_option (gdb::array_view<const option_def_group> options_group,
 		      complete_on_options (options_group,
 					   completion->tracker,
 					   arg, completion->word);
-		      return {};
+		      return gdb::nullopt;
 		    }
 
 		  error (_("Ambiguous option at: -%s"), arg);
@@ -237,14 +237,14 @@  parse_option (gdb::array_view<const option_def_group> options_group,
       if (have_delimiter || mode != PROCESS_OPTIONS_UNKNOWN_IS_OPERAND)
 	error (_("Unrecognized option at: %s"), *args);
 
-      return {};
+      return gdb::nullopt;
     }
 
   if (completion != nullptr && arg[len] == '\0')
     {
       complete_on_options (options_group, completion->tracker,
 			   arg, completion->word);
-      return {};
+      return gdb::nullopt;
     }
 
   *args += 1 + len;
@@ -347,7 +347,7 @@  parse_option (gdb::array_view<const option_def_group> options_group,
 		};
 		complete_on_enum (completion->tracker, all_boolean_enums,
 				  val_str, val_str);
-		return {};
+		return gdb::nullopt;
 	      }
 	  }
 
@@ -370,13 +370,13 @@  parse_option (gdb::array_view<const option_def_group> options_group,
 		  (make_unique_xstrdup ("NUMBER"));
 		completion->tracker.add_completion
 		  (make_unique_xstrdup ("unlimited"));
-		return {};
+		return gdb::nullopt;
 	      }
 	    else if (startswith ("unlimited", *args))
 	      {
 		completion->tracker.add_completion
 		  (make_unique_xstrdup ("unlimited"));
-		return {};
+		return gdb::nullopt;
 	      }
 	  }
 
@@ -403,7 +403,7 @@  parse_option (gdb::array_view<const option_def_group> options_group,
 		complete_on_enum (completion->tracker,
 				  match->enums, *args, *args);
 		if (completion->tracker.have_completions ())
-		  return {};
+		  return gdb::nullopt;
 
 		/* If we don't have completions, let the
 		   non-completion path throw on invalid enum value
@@ -448,7 +448,7 @@  parse_option (gdb::array_view<const option_def_group> options_group,
       gdb_assert_not_reached (_("option type not supported"));
     }
 
-  return {};
+  return gdb::nullopt;
 }
 
 /* See cli-option.h.  */
diff --git a/gdb/dwarf2/die.h b/gdb/dwarf2/die.h
index 5522ebdf31..9f3a877e73 100644
--- a/gdb/dwarf2/die.h
+++ b/gdb/dwarf2/die.h
@@ -45,7 +45,7 @@  struct die_info
 	  /* If both exist, just use the first one.  */
 	  return DW_UNSND (&attrs[i]);
 	}
-    return gdb::optional<ULONGEST> ();
+    return gdb::nullopt;
   }
 
   /* Return range lists base of the compile unit, which, if exists, is
diff --git a/gdb/dwarf2/read.c b/gdb/dwarf2/read.c
index 719051bc5b..13c485b15d 100644
--- a/gdb/dwarf2/read.c
+++ b/gdb/dwarf2/read.c
@@ -3586,7 +3586,8 @@  dw2_expand_symtabs_for_function (struct objfile *objfile,
   struct dw2_symtab_iterator iter;
   struct dwarf2_per_cu_data *per_cu;
 
-  dw2_symtab_iter_init (&iter, dwarf2_per_objfile, {}, VAR_DOMAIN, func_name);
+  dw2_symtab_iter_init (&iter, dwarf2_per_objfile, gdb::nullopt, VAR_DOMAIN,
+			func_name);
 
   while ((per_cu = dw2_symtab_iter_next (&iter)) != NULL)
     dw2_instantiate_symtab (per_cu, false);
@@ -5624,7 +5625,7 @@  dw2_debug_names_expand_symtabs_for_function (struct objfile *objfile,
     {
       const mapped_debug_names &map = *dwarf2_per_objfile->debug_names_table;
 
-      dw2_debug_names_iterator iter (map, {}, VAR_DOMAIN, func_name);
+      dw2_debug_names_iterator iter (map, gdb::nullopt, VAR_DOMAIN, func_name);
 
       struct dwarf2_per_cu_data *per_cu;
       while ((per_cu = iter.next ()) != NULL)
@@ -6760,7 +6761,7 @@  lookup_dwo_id (struct dwarf2_cu *cu, struct die_info* comp_unit_die)
   struct attribute *attr;
   attr = dwarf2_attr (comp_unit_die, DW_AT_GNU_dwo_id, cu);
   if (attr == nullptr)
-    return gdb::optional<ULONGEST> ();
+    return gdb::nullopt;
   return DW_UNSND (attr);
 }
 
diff --git a/gdb/fbsd-tdep.c b/gdb/fbsd-tdep.c
index acf787c706..b8275814af 100644
--- a/gdb/fbsd-tdep.c
+++ b/gdb/fbsd-tdep.c
@@ -694,7 +694,7 @@  fbsd_make_note_desc (enum target_object object, uint32_t structsize)
   gdb::optional<gdb::byte_vector> buf =
     target_read_alloc (current_top_target (), object, NULL);
   if (!buf || buf->empty ())
-    return {};
+    return gdb::nullopt;
 
   if (structsize == 0)
     return buf;
diff --git a/gdb/inf-child.c b/gdb/inf-child.c
index 4833094889..3420398f82 100644
--- a/gdb/inf-child.c
+++ b/gdb/inf-child.c
@@ -381,13 +381,13 @@  inf_child_target::fileio_readlink (struct inferior *inf, const char *filename,
   if (len < 0)
     {
       *target_errno = host_to_fileio_error (errno);
-      return {};
+      return gdb::nullopt;
     }
 
   return std::string (buf, len);
 #else
   *target_errno = FILEIO_ENOSYS;
-  return {};
+  return gdb::nullopt;
 #endif
 }
 
diff --git a/gdb/linux-nat.c b/gdb/linux-nat.c
index 0a2bfdc57d..8de9501639 100644
--- a/gdb/linux-nat.c
+++ b/gdb/linux-nat.c
@@ -4480,7 +4480,7 @@  linux_nat_target::fileio_readlink (struct inferior *inf, const char *filename,
   if (len < 0)
     {
       *target_errno = host_to_fileio_error (errno);
-      return {};
+      return gdb::nullopt;
     }
 
   return std::string (buf, len);
diff --git a/gdb/python/python.c b/gdb/python/python.c
index 67f362b852..c241a09f71 100644
--- a/gdb/python/python.c
+++ b/gdb/python/python.c
@@ -1120,35 +1120,35 @@  static gdb::optional<std::string>
 gdbpy_colorize (const std::string &filename, const std::string &contents)
 {
   if (!gdb_python_initialized)
-    return {};
+    return gdb::nullopt;
 
   gdbpy_enter enter_py (get_current_arch (), current_language);
 
   if (gdb_python_module == nullptr
       || !PyObject_HasAttrString (gdb_python_module, "colorize"))
-    return {};
+    return gdb::nullopt;
 
   gdbpy_ref<> hook (PyObject_GetAttrString (gdb_python_module, "colorize"));
   if (hook == nullptr)
     {
       gdbpy_print_stack ();
-      return {};
+      return gdb::nullopt;
     }
 
   if (!PyCallable_Check (hook.get ()))
-    return {};
+    return gdb::nullopt;
 
   gdbpy_ref<> fname_arg (PyString_FromString (filename.c_str ()));
   if (fname_arg == nullptr)
     {
       gdbpy_print_stack ();
-      return {};
+      return gdb::nullopt;
     }
   gdbpy_ref<> contents_arg (PyString_FromString (contents.c_str ()));
   if (contents_arg == nullptr)
     {
       gdbpy_print_stack ();
-      return {};
+      return gdb::nullopt;
     }
 
   gdbpy_ref<> result (PyObject_CallFunctionObjArgs (hook.get (),
@@ -1158,17 +1158,17 @@  gdbpy_colorize (const std::string &filename, const std::string &contents)
   if (result == nullptr)
     {
       gdbpy_print_stack ();
-      return {};
+      return gdb::nullopt;
     }
 
   if (!gdbpy_is_string (result.get ()))
-    return {};
+    return gdb::nullopt;
 
   gdbpy_ref<> unic = python_string_to_unicode (result.get ());
   if (unic == nullptr)
     {
       gdbpy_print_stack ();
-      return {};
+      return gdb::nullopt;
     }
   gdbpy_ref<> host_str (PyUnicode_AsEncodedString (unic.get (),
 						   host_charset (),
@@ -1176,7 +1176,7 @@  gdbpy_colorize (const std::string &filename, const std::string &contents)
   if (host_str == nullptr)
     {
       gdbpy_print_stack ();
-      return {};
+      return gdb::nullopt;
     }
 
   return std::string (PyBytes_AsString (host_str.get ()));
diff --git a/gdb/remote.c b/gdb/remote.c
index 312a03c8fb..b910f7e533 100644
--- a/gdb/remote.c
+++ b/gdb/remote.c
@@ -12257,7 +12257,7 @@  remote_target::fileio_readlink (struct inferior *inf, const char *filename,
   int read_len;
 
   if (remote_hostio_set_filesystem (inf, remote_errno) != 0)
-    return {};
+    return gdb::nullopt;
 
   remote_buffer_add_string (&p, &left, "vFile:readlink:");
 
@@ -12269,7 +12269,7 @@  remote_target::fileio_readlink (struct inferior *inf, const char *filename,
 				    &attachment_len);
 
   if (len < 0)
-    return {};
+    return gdb::nullopt;
 
   std::string ret (len, '\0');
 
diff --git a/gdb/solib-aix.c b/gdb/solib-aix.c
index 125c8960c2..16cf235a20 100644
--- a/gdb/solib-aix.c
+++ b/gdb/solib-aix.c
@@ -108,7 +108,7 @@  solib_aix_parse_libraries (const char *library)
                  "at compile time"));
     }
 
-  return {};
+  return gdb::nullopt;
 }
 
 #else /* HAVE_LIBEXPAT */
@@ -215,7 +215,7 @@  solib_aix_parse_libraries (const char *library)
 			   library_list_elements, library, &result) == 0)
     return result;
 
-  return {};
+  return gdb::nullopt;
 }
 
 #endif /* HAVE_LIBEXPAT */
diff --git a/gdb/solib-svr4.c b/gdb/solib-svr4.c
index 19d1105ae9..6df604cc2f 100644
--- a/gdb/solib-svr4.c
+++ b/gdb/solib-svr4.c
@@ -429,13 +429,13 @@  read_program_header (int type, int *p_arch_size, CORE_ADDR *base_addr)
 
   /* Get required auxv elements from target.  */
   if (target_auxv_search (current_top_target (), AT_PHDR, &at_phdr) <= 0)
-    return {};
+    return gdb::nullopt;
   if (target_auxv_search (current_top_target (), AT_PHENT, &at_phent) <= 0)
-    return {};
+    return gdb::nullopt;
   if (target_auxv_search (current_top_target (), AT_PHNUM, &at_phnum) <= 0)
-    return {};
+    return gdb::nullopt;
   if (!at_phdr || !at_phnum)
-    return {};
+    return gdb::nullopt;
 
   /* Determine ELF architecture type.  */
   if (at_phent == sizeof (Elf32_External_Phdr))
@@ -443,7 +443,7 @@  read_program_header (int type, int *p_arch_size, CORE_ADDR *base_addr)
   else if (at_phent == sizeof (Elf64_External_Phdr))
     arch_size = 64;
   else
-    return {};
+    return gdb::nullopt;
 
   /* Find the requested segment.  */
   if (type == -1)
@@ -463,7 +463,7 @@  read_program_header (int type, int *p_arch_size, CORE_ADDR *base_addr)
 
 	  if (target_read_memory (at_phdr + i * sizeof (phdr),
 				  (gdb_byte *)&phdr, sizeof (phdr)))
-	    return {};
+	    return gdb::nullopt;
 
 	  p_type = extract_unsigned_integer ((gdb_byte *) phdr.p_type,
 					     4, byte_order);
@@ -480,7 +480,7 @@  read_program_header (int type, int *p_arch_size, CORE_ADDR *base_addr)
 	}
 
       if (i == at_phnum)
-	return {};
+	return gdb::nullopt;
 
       /* Retrieve address and size.  */
       sect_addr = extract_unsigned_integer ((gdb_byte *)phdr.p_vaddr,
@@ -500,7 +500,7 @@  read_program_header (int type, int *p_arch_size, CORE_ADDR *base_addr)
 
 	  if (target_read_memory (at_phdr + i * sizeof (phdr),
 				  (gdb_byte *)&phdr, sizeof (phdr)))
-	    return {};
+	    return gdb::nullopt;
 
 	  p_type = extract_unsigned_integer ((gdb_byte *) phdr.p_type,
 					     4, byte_order);
@@ -517,7 +517,7 @@  read_program_header (int type, int *p_arch_size, CORE_ADDR *base_addr)
 	}
 
       if (i == at_phnum)
-	return {};
+	return gdb::nullopt;
 
       /* Retrieve address and size.  */
       sect_addr = extract_unsigned_integer ((gdb_byte *)phdr.p_vaddr,
@@ -539,7 +539,7 @@  read_program_header (int type, int *p_arch_size, CORE_ADDR *base_addr)
   /* Read in requested program header.  */
   gdb::byte_vector buf (sect_size);
   if (target_read_memory (sect_addr, buf.data (), sect_size))
-    return {};
+    return gdb::nullopt;
 
   if (p_arch_size)
     *p_arch_size = arch_size;
@@ -2510,12 +2510,12 @@  read_program_headers_from_bfd (bfd *abfd)
   Elf_Internal_Ehdr *ehdr = elf_elfheader (abfd);
   int phdrs_size = ehdr->e_phnum * ehdr->e_phentsize;
   if (phdrs_size == 0)
-    return {};
+    return gdb::nullopt;
 
   gdb::byte_vector buf (phdrs_size);
   if (bfd_seek (abfd, ehdr->e_phoff, SEEK_SET) != 0
       || bfd_bread (buf.data (), phdrs_size, abfd) != phdrs_size)
-    return {};
+    return gdb::nullopt;
 
   return buf;
 }
diff --git a/gdb/symtab.h b/gdb/symtab.h
index 05e6a311b8..3a1200cfa1 100644
--- a/gdb/symtab.h
+++ b/gdb/symtab.h
@@ -471,8 +471,7 @@  struct general_symbol_info
      copy_name must be set to true.  */
   void compute_and_set_names (gdb::string_view linkage_name, bool copy_name,
 			      struct objfile_per_bfd_storage *per_bfd,
-			      gdb::optional<hashval_t> hash
-			        = gdb::optional<hashval_t> ());
+			      gdb::optional<hashval_t> hash = gdb::nullopt);
 
   /* Name of the symbol.  This is a required field.  Storage for the
      name is allocated on the objfile_obstack for the associated
diff --git a/gdb/target.c b/gdb/target.c
index 6982a806e3..67e26fb46d 100644
--- a/gdb/target.c
+++ b/gdb/target.c
@@ -1781,7 +1781,7 @@  target_read_alloc_1 (struct target_ops *ops, enum target_object object,
       else if (status != TARGET_XFER_OK)
 	{
 	  /* An error occurred.  */
-	  return {};
+	  return gdb::nullopt;
 	}
 
       buf_pos += xfered_len;
@@ -1809,7 +1809,7 @@  target_read_stralloc (struct target_ops *ops, enum target_object object,
     = target_read_alloc_1<char> (ops, object, annex);
 
   if (!buf)
-    return {};
+    return gdb::nullopt;
 
   if (buf->empty () || buf->back () != '\0')
     buf->push_back ('\0');
@@ -2536,7 +2536,7 @@  target_get_osdata (const char *type)
     t = find_default_run_target ("get OS data");
 
   if (!t)
-    return {};
+    return gdb::nullopt;
 
   return target_read_stralloc (t, TARGET_OBJECT_OSDATA, type);
 }
@@ -2772,7 +2772,7 @@  target_ops::fileio_readlink (struct inferior *inf, const char *filename,
 			     int *target_errno)
 {
   *target_errno = FILEIO_ENOSYS;
-  return {};
+  return gdb::nullopt;
 }
 
 /* Helper for target_fileio_open and
@@ -2984,7 +2984,7 @@  target_fileio_readlink (struct inferior *inf, const char *filename,
     }
 
   *target_errno = FILEIO_ENOSYS;
-  return {};
+  return gdb::nullopt;
 }
 
 /* Like scoped_fd, but specific to target fileio.  */
diff --git a/gdb/xml-support.c b/gdb/xml-support.c
index c906b69fda..5177315be5 100644
--- a/gdb/xml-support.c
+++ b/gdb/xml-support.c
@@ -984,7 +984,7 @@  xml_fetch_content_from_file (const char *filename, void *baton)
     file = gdb_fopen_cloexec (filename, FOPEN_RB);
 
   if (file == NULL)
-    return {};
+    return gdb::nullopt;
 
   /* Read in the whole file.  */
 
@@ -1001,7 +1001,7 @@  xml_fetch_content_from_file (const char *filename, void *baton)
       || ferror (file.get ()))
     {
       warning (_("Read error from \"%s\""), filename);
-      return {};
+      return gdb::nullopt;
     }
 
   text.back () = '\0';
diff --git a/gdb/xml-tdesc.c b/gdb/xml-tdesc.c
index 6f108a4e3e..23b59e9d79 100644
--- a/gdb/xml-tdesc.c
+++ b/gdb/xml-tdesc.c
@@ -730,12 +730,12 @@  target_fetch_description_xml (struct target_ops *ops)
 		 "disabled at compile time"));
     }
 
-  return {};
+  return gdb::nullopt;
 #else
   gdb::optional<gdb::char_vector>
     tdesc_str = fetch_available_features_from_target ("target.xml", ops);
   if (!tdesc_str)
-    return {};
+    return gdb::nullopt;
 
   std::string output;
   if (!xml_process_xincludes (output,
@@ -744,7 +744,7 @@  target_fetch_description_xml (struct target_ops *ops)
 			      fetch_available_features_from_target, ops, 0))
     {
       warning (_("Could not load XML target description; ignoring"));
-      return {};
+      return gdb::nullopt;
     }
   return output;
 #endif
@@ -758,6 +758,6 @@  string_read_description_xml (const char *xml)
   return tdesc_parse_xml (xml, [] (const char *href, void *baton)
     {
       error (_("xincludes are unsupported with this method"));
-      return gdb::optional<gdb::char_vector> ();
+      return gdb::nullopt;
     }, nullptr);
 }
diff --git a/gdbsupport/gdb_optional.h b/gdbsupport/gdb_optional.h
index 02a87f6ee4..d1e70e4be9 100644
--- a/gdbsupport/gdb_optional.h
+++ b/gdbsupport/gdb_optional.h
@@ -25,12 +25,17 @@ 
 namespace gdb
 {
 
+struct nullopt_t {
+    explicit constexpr nullopt_t (int) {}
+};
+
 struct in_place_t
 {
   explicit in_place_t () = default;
 };
 
 constexpr gdb::in_place_t in_place {};
+constexpr nullopt_t nullopt {42};
 
 /* This class attempts to be a compatible subset of std::optional,
    which is slated to be available in C++17.  This class optionally
@@ -55,6 +60,9 @@  class optional
   constexpr optional ()
     : m_dummy ()
   {}
+  constexpr optional (nullopt_t)
+    : m_dummy ()
+  {}
 
   template<typename... Args>
   constexpr optional (in_place_t, Args &&... args)
@@ -93,6 +101,12 @@  class optional
 
   /* Assignment operators.  */
 
+  optional &
+  operator= (nullopt_t)
+  {
+    reset ();
+  }
+
   optional &
   operator= (const optional &other)
   {