4 files changed, 21 insertions, 13 deletions
diff --git a/crates/shirabe/src/io/buffer_io.rs b/crates/shirabe/src/io/buffer_io.rs
index ed87b71..291b64b 100644
--- a/crates/shirabe/src/io/buffer_io.rs
+++ b/crates/shirabe/src/io/buffer_io.rs
@@ -245,7 +245,11 @@ impl crate::io::IOInterfaceMutable for BufferIO {
     }
 }
 
-impl crate::io::IOInterface for BufferIO {}
+impl crate::io::IOInterface for BufferIO {
+    fn as_any(&self) -> &dyn std::any::Any {
+        self
+    }
+}
 
 impl crate::io::BaseIO for BufferIO {
     fn authentications(
diff --git a/crates/shirabe/src/io/console_io.rs b/crates/shirabe/src/io/console_io.rs
index 2b1c334..b69b68b 100644
--- a/crates/shirabe/src/io/console_io.rs
+++ b/crates/shirabe/src/io/console_io.rs
@@ -142,10 +142,7 @@ impl ConsoleIO {
             messages
         };
 
-        if true == stderr && self.output.is_console_output_interface() {
-            // TODO(phase-b): downcast Box<dyn OutputInterface> to ConsoleOutputInterface
-            let console_output: &dyn ConsoleOutputInterface =
-                todo!("downcast self.output to ConsoleOutputInterface");
+        if stderr && let Some(console_output) = self.output.as_console_output_interface() {
             console_output.get_error_output().write(
                 &Self::to_string_list(&messages).join(if newline { "\n" } else { "" }),
                 newline,
@@ -263,11 +260,8 @@ impl ConsoleIO {
     }
 
     fn get_error_output(&self) -> &dyn OutputInterface {
-        if self.output.is_console_output_interface() {
-            // TODO(phase-b): downcast Box<dyn OutputInterface> to ConsoleOutputInterface
-            return todo!(
-                "downcast self.output to ConsoleOutputInterface and call get_error_output()"
-            );
+        if let Some(console_output) = self.output.as_console_output_interface() {
+            return console_output.get_error_output();
         }
 
         &*self.output
@@ -677,7 +671,11 @@ impl IOInterfaceMutable for ConsoleIO {
     }
 }
 
-impl IOInterface for ConsoleIO {}
+impl IOInterface for ConsoleIO {
+    fn as_any(&self) -> &dyn std::any::Any {
+        self
+    }
+}
 
 impl BaseIO for ConsoleIO {
     fn authentications(
diff --git a/crates/shirabe/src/io/io_interface.rs b/crates/shirabe/src/io/io_interface.rs
index aac1488..e48f9ce 100644
--- a/crates/shirabe/src/io/io_interface.rs
+++ b/crates/shirabe/src/io/io_interface.rs
@@ -149,7 +149,9 @@ pub trait IOInterfaceMutable {
 
 // PHP `IOInterface`. This is the type used for the shared trait object
 // `dyn IOInterface`; its vtable carries both the immutable and mutable methods.
-pub trait IOInterface: IOInterfaceImmutable + IOInterfaceMutable {}
+pub trait IOInterface: IOInterfaceImmutable + IOInterfaceMutable {
+    fn as_any(&self) -> &dyn std::any::Any;
+}
 
 // Shared-ownership handle for a PHP IO instance (reference semantics). It
 // exposes only the immutable surface; mutating methods (`set_authentication`,
diff --git a/crates/shirabe/src/io/null_io.rs b/crates/shirabe/src/io/null_io.rs
index b0e11f1..85a5fa5 100644
--- a/crates/shirabe/src/io/null_io.rs
+++ b/crates/shirabe/src/io/null_io.rs
@@ -138,7 +138,11 @@ impl IOInterfaceMutable for NullIO {
     }
 }
 
-impl IOInterface for NullIO {}
+impl IOInterface for NullIO {
+    fn as_any(&self) -> &dyn std::any::Any {
+        self
+    }
+}
 
 impl BaseIO for NullIO {
     fn authentications(