Added a test. It seems to work

Author: epsilon-phase

hdr/sqlite_modern_cpp/type_wrapper.h

@@ -167,9 +167,21 @@ namespace sqlite {
 		sqlite3_result_null(db);
 	}
 #ifdef MODERN_SQLITE_STD_VARIANT_SUPPORT
+	template<>
+	struct has_sqlite_type<std::monostate, SQLITE_NULL, void> : std::true_type {};
+  
+	inline std::monostate get_col_from_db(sqlite3_stmt* stmt, int inx, result_type<std::monostate>) {
+    if(sqlite3_column_type(stmt,inx) != SQLITE_NULL) throw 1;
+    return std::monostate();
+  }
+	inline std::monostate get_val_from_db(sqlite3_value *value, result_type<std::monostate>) {
+    return std::monostate();
+  }
+
 	inline int bind_col_in_db(sqlite3_stmt* stmt, int inx, std::monostate) {
 		return sqlite3_bind_null(stmt, inx);
 	}
+
 	inline void store_result_in_db(sqlite3_context* db, std::monostate) {
 		sqlite3_result_null(db);
 	}

tests/variant.cc

@@ -29,4 +29,25 @@ TEST_CASE("std::variant works", "[variant]") {
     db << "SELECT 0.0" >> v;
     REQUIRE(std::get<2>(v)); 
 }
+TEST_CASE("std::monostate is a nullptr substitute", "[variant]"){
+    database db(":memory:");
+    db << "CREATE TABLE foo (a);";
+
+    std::variant<std::monostate,std::string> v;
+    v=std::monostate();
+    db << "INSERT INTO foo values(?)" << v;
+    db << "INSERT INTO foo values(?)" << "This isn't a monostate!";
+    bool found_null=false,
+         found_string=false;
+    db << "select * from foo" >> [&found_null,&found_string](std::variant<std::monostate,std::string> z){
+      if(z.index()==0){
+        found_null=true;
+      }else{
+        found_string=true;
+      };
+    };
+    REQUIRE((found_null&&found_string));
+    db << "SELECT NULL" >> v;
+    REQUIRE(v.index()==0);
+}
 #endif