Make cpp2util.h warning-clean too, on all major compilers (/W4, -Wall -Wextra)

And increase MSVC warning-clean level to /W4 for everything

Author: hsutter

include/cpp2util.h

@@ -351,7 +351,7 @@ struct {
     }
 } unique;
 
-struct {
+[[maybe_unused]] struct {
     template<typename T>
     [[nodiscard]] auto cpp2_new(auto&& ...args) const -> std::shared_ptr<T> {
         return std::make_shared<T>(std::forward<decltype(args)>(args)...);
@@ -1123,7 +1123,19 @@ class c_raii {
 };
 
 inline auto fopen( const char* filename, const char* mode ) {
+
+    //  Suppress annoying deprecation warning about fopen
+    #ifdef _MSC_VER
+        #pragma warning( push )
+        #pragma warning( disable : 4996 )
+    #endif
+
     auto x = std::fopen(filename, mode);
+
+    #ifdef _MSC_VER
+        #pragma warning( pop )
+    #endif
+
     if (!x) {
         throw std::make_error_condition(std::errc::no_such_file_or_directory);
     }

source/parse.h

@@ -3287,9 +3287,9 @@ class parser
 
             if (body->statements.empty() || body->statements.back()->statement.index() != statement_node::return_)
             {
-                auto pos = body->statements.back()->position();
-                ++pos.lineno;
-                generated_tokens_->emplace_back( "return", pos, lexeme::Keyword);
+                auto last_pos = body->statements.back()->position();
+                ++last_pos.lineno;
+                generated_tokens_->emplace_back( "return", last_pos, lexeme::Keyword);
 
                 auto ret = std::make_unique<return_statement_node>();
                 ret->identifier = &generated_tokens_->back();