fable: Force compilation stop when C++ standard less than 17

Author: cassava

fable/include/fable/conf.hpp

@@ -57,6 +57,7 @@
 
 #include <fmt/format.h>  // for fmt::format
 
+#include <fable/fable_fwd.hpp>
 #include <fable/json.hpp>  // for Json
 
 namespace fable {

fable/include/fable/confable.hpp

@@ -25,6 +25,7 @@
 
 #include <memory>  // for unique_ptr<>
 
+#include <fable/fable_fwd.hpp>
 #include <fable/json.hpp>    // for Json
 #include <fable/conf.hpp>    // for Conf
 

fable/include/fable/enum.hpp

@@ -27,6 +27,7 @@
 #include <type_traits>  // for enable_if_t<>, is_enum<>
 #include <utility>      // for make_pair, move
 
+#include <fable/fable_fwd.hpp>
 #include <fable/json.hpp>
 
 /**

fable/include/fable/environment.hpp

@@ -29,6 +29,8 @@
 #include <utility>  // for pair<>, move, make_pair
 #include <optional> // for optional<>
 
+#include <fable/fable_fwd.hpp>
+
 namespace fable {
 
 class Environment {

fable/include/fable/error.hpp

@@ -27,6 +27,7 @@
 #include <fmt/format.h>  // for format
 
 #include <fable/conf.hpp>  // for Conf
+#include <fable/fable_fwd.hpp>
 #include <fable/json.hpp>  // for Json
 
 namespace fable {

fable/include/fable/fable.hpp

@@ -32,5 +32,4 @@
 #include <fable/error.hpp>
 #include <fable/json.hpp>
 #include <fable/utility/memory.hpp>
-#include <fable/utility/optional.hpp>
 #include <fable/schema.hpp>

fable/include/fable/fable_fwd.hpp

@@ -19,6 +19,11 @@
  * \file fable/fable_fwd.hpp
  */
 
+#if __cplusplus < 201703L
+#error "the fable library requires a minimum standard of C++17"
+#include <force_compiler_to_stop_here>
+#endif
+
 #include <string>
 
 #include <nlohmann/detail/value_t.hpp>

fable/include/fable/json.hpp

@@ -28,6 +28,8 @@
 
 #include <nlohmann/json.hpp>  // for Json
 
+#include <fable/fable_fwd.hpp>
+
 namespace fable {
 
 /**

fable/include/fable/schema.hpp

@@ -120,6 +120,7 @@
 #include <utility>      // for move
 #include <vector>       // for vector<>
 
+#include <fable/fable_fwd.hpp>
 #include <fable/schema/array.hpp>      // for Array<>
 #include <fable/schema/boolean.hpp>    // for Boolean
 #include <fable/schema/confable.hpp>   // for FromConfable

fable/include/fable/utility/gtest.hpp

@@ -25,6 +25,7 @@
 
 #include <gtest/gtest.h>  // for ASSERT_THROW, ASSERT_EQ
 
+#include <fable/fable_fwd.hpp>
 #include <fable/conf.hpp>      // for Conf, Json
 #include <fable/confable.hpp>  // for Confable
 #include <fable/schema.hpp>    // for Schema