Replace nonstd::optional by C++17 std::optional

Removes third-party code that is no longer necessary now that we use C++
17 as build standard. Future uses of optionalt should instead use
std::optional directly.

Author: tautschnig

CODEOWNERS

@@ -23,7 +23,6 @@
 /src/json-symtab-language/ @martin-cs
 /src/langapi/ @kroening @tautschnig @peterschrammel
 /src/xmllang/ @kroening @tautschnig @peterschrammel
-/src/nonstd/ @peterschrammel
 /src/solvers/flattening @martin-cs @kroening @tautschnig @peterschrammel
 /src/solvers/floatbv @martin-cs @kroening
 /src/solvers/miniBDD @tautschnig @kroening

doc/architectural/folder-walkthrough.md

@@ -51,7 +51,6 @@ containing the code for a different part of the system.
   * \ref xmllang
   * \ref util
   * \ref miniz
-  * \ref nonstd
 
 In the top level of `src` there are only a few files:
 
@@ -167,7 +166,6 @@ digraph directory_dependencies {
     big_int [label = "big-int", URL = "\ref big-int"];
     miniz [URL = "\ref miniz"];
     util [URL = "\ref util"];
-    nonstd [URL = "\ref nonstd"];
     json [URL = "\ref json"];
     xmllang [URL = "\ref xmllang"];
     assembler [URL = "\ref assembler"];
@@ -203,6 +201,5 @@ digraph directory_dependencies {
   xmllang -> util;
   assembler -> util;
   util -> big_int;
-  util -> nonstd;
 }
 \enddot

jbmc/src/java_bytecode/assignments_from_json.cpp

@@ -799,7 +799,13 @@ static code_with_references_listt assign_reference_from_json(
   const std::string &id = has_enum_type(expr, info.symbol_table)
                             ? get_enum_id(expr, json, info.symbol_table)
                             : get_id_or_reference_value(json);
+// Work around spurious GCC 13 warning about references being uninitialised.
+#pragma GCC diagnostic push
+#ifndef __clang__
+#  pragma GCC diagnostic ignored "-Wmaybe-uninitialized"
+#endif
   auto get_reference_result = get_or_create_reference(expr, id, info);
+#pragma GCC diagnostic pop
   const bool is_new_id = get_reference_result.newly_allocated;
   object_creation_referencet &reference =
     get_reference_result.reference->second;
@@ -868,7 +874,13 @@ code_with_references_listt assign_from_json_rec(
     // has_enum_type(expr, info.symbol_table) once tracking reference-equality
     // across different classes has been implemented.
     {
+// Work around spurious GCC 13 warning about references being uninitialised.
+#pragma GCC diagnostic push
+#ifndef __clang__
+#  pragma GCC diagnostic ignored "-Wmaybe-uninitialized"
+#endif
       return assign_reference_from_json(expr, json, type_from_array, info);
+#pragma GCC diagnostic pop
     }
     else if(is_java_array_type(expr.type()))
     {

jbmc/src/java_bytecode/ci_lazy_methods.h

@@ -82,7 +82,7 @@ class method_bytecodet
   {
     const auto it = map.find(method_id);
     if(it == map.end())
-      return opt_reft();
+      return std::nullopt;
     return std::cref(it->second);
   }
 };

src/goto-instrument/contracts/dynamic-frames/dfcc_cfg_info.cpp

@@ -138,7 +138,7 @@ dfcc_loop_infot::find_head(goto_programt &goto_program) const
 optionalt<goto_programt::targett>
 dfcc_loop_infot::find_latch(goto_programt &goto_program) const
 {
-  optionalt<goto_programt::targett> result = nullopt;
+  optionalt<goto_programt::targett> result = std::nullopt;
   for(auto target = goto_program.instructions.begin();
       target != goto_program.instructions.end();
       target++)
@@ -728,7 +728,7 @@ dfcc_cfg_infot::get_outer_loop_identifier(const std::size_t loop_id) const
     }
   }
   // return nullopt for loops that are not nested in other loops
-  return nullopt;
+  return std::nullopt;
 }
 
 bool dfcc_cfg_infot::is_valid_loop_or_top_level_id(const std::size_t id) const

src/goto-instrument/contracts/dynamic-frames/dfcc_lift_memory_predicates.cpp

@@ -331,7 +331,7 @@ void dfcc_lift_memory_predicatest::lift_parameters_and_update_body(
     // rewrite all occurrences of lifted parameters
     instruction.transform([&replace_lifted_params](exprt expr) {
       const bool changed = !replace_lifted_params.replace(expr);
-      return changed ? optionalt<exprt>{expr} : nullopt;
+      return changed ? optionalt<exprt>{expr} : std::nullopt;
     });
 
     // add address-of to all arguments expressions passed in lifted position to

src/goto-programs/link_goto_model.cpp

@@ -217,7 +217,7 @@ void finalize_linking(
         {
           instruction.transform([&object_type_updates](exprt expr) {
             const bool changed = !object_type_updates.replace(expr);
-            return changed ? optionalt<exprt>{expr} : nullopt;
+            return changed ? optionalt<exprt>{expr} : std::nullopt;
           });
         }
       }