Gracefully handle incorrect command aliases

Even if the aliased target is a helper method or does not exist, IRB
should not crash.

This commit warns users in such cases and treat the input as normal expression.

lib/irb.rb

@@ -281,13 +281,19 @@ def build_statement(code)
       end
 
       code = code.dup.force_encoding(@context.io.encoding)
+
       if (command, arg = @context.parse_command(code))
-        command_class = Command.load_command(command)
-        Statement::Command.new(code, command_class, arg)
-      else
-        is_assignment_expression = @scanner.assignment_expression?(code, local_variables: @context.local_variables)
-        Statement::Expression.new(code, is_assignment_expression)
+        if (command_class = Command.load_command(command))
+          return Statement::Command.new(code, command_class, arg)
+        elsif (helper_class = HelperMethod.helper_methods[command])
+          warn "Using command alias for helper method '#{command}' is not supported"
+        else
+          warn "Command '#{command}' does not exist"
+        end
       end
+
+      is_assignment_expression = @scanner.assignment_expression?(code, local_variables: @context.local_variables)
+      Statement::Expression.new(code, is_assignment_expression)
     end
 
     def command?(code)

test/irb/command/test_command_aliasing.rb

@@ -0,0 +1,40 @@
+# frozen_string_literal: true
+
+require "tempfile"
+require_relative "../helper"
+
+module TestIRB
+  class CommandAliasingTest < IntegrationTestCase
+    def setup
+      super
+      write_rc <<~RUBY
+        IRB.conf[:COMMAND_ALIASES] = {
+          :c => :conf, # alias to helper method
+          :f => :foo
+        }
+      RUBY
+
+      write_ruby <<~'RUBY'
+        binding.irb
+      RUBY
+    end
+
+    def test_aliasing_to_helper_method_triggers_warning
+      out = run_ruby_file do
+        type "c"
+        type "exit"
+      end
+      assert_include(out, "Using command alias for helper method 'conf' is not supported")
+      assert_not_include(out, "Maybe IRB bug!")
+    end
+
+    def test_incorrect_alias_triggers_warning
+      out = run_ruby_file do
+        type "f"
+        type "exit"
+      end
+      assert_include(out, "Command 'foo' does not exist")
+      assert_not_include(out, "Maybe IRB bug!")
+    end
+  end
+end