[LLDB]Provide clearer error message for invalid commands.

lldb/include/lldb/Interpreter/CommandObjectMultiword.h

@@ -70,6 +70,8 @@ class CommandObjectMultiword : public CommandObject {
     return m_subcommand_dict;
   }
 
+  std::string GetSubcommandsHintText();
+
   CommandObject::CommandMap m_subcommand_dict;
   bool m_can_be_removed;
 };

lldb/source/Commands/CommandObjectMultiword.cpp

@@ -194,28 +194,54 @@ void CommandObjectMultiword::Execute(const char *args_string,
 
   std::string error_msg;
   const size_t num_subcmd_matches = matches.GetSize();
-  if (num_subcmd_matches > 0)
+  if (num_subcmd_matches > 0) {
     error_msg.assign("ambiguous command ");
-  else
-    error_msg.assign("invalid command ");
-
-  error_msg.append("'");
-  error_msg.append(std::string(GetCommandName()));
-  error_msg.append(" ");
-  error_msg.append(std::string(sub_command));
-  error_msg.append("'.");
+    error_msg.append("'");
+    error_msg.append(std::string(GetCommandName()));
+    error_msg.append(" ");
+    error_msg.append(std::string(sub_command));
+    error_msg.append("'.");
 
-  if (num_subcmd_matches > 0) {
     error_msg.append(" Possible completions:");
     for (const std::string &match : matches) {
       error_msg.append("\n\t");
       error_msg.append(match);
     }
+  } else {
+    // Try to offer some alternatives to help correct the command.
+    error_msg.assign(
+        llvm::Twine("\"" + sub_command + "\" is not a valid subcommand of \"" +
+                    GetCommandName() + "\"." + GetSubcommandsHintText() +
+                    " Use \"help " + GetCommandName() + "\" to find out more.")
+            .str());
   }
   error_msg.append("\n");
   result.AppendRawError(error_msg.c_str());
 }
 
+std::string CommandObjectMultiword::GetSubcommandsHintText() {
+  if (m_subcommand_dict.empty())
+    return "";
+  const size_t maxCount = 5;
+  size_t i = 0;
+  std::string buffer = " Valid subcommand";
+  buffer.append(m_subcommand_dict.size() > 1 ? "s are:" : " is");
+  CommandMap::iterator pos;
+  for (pos = m_subcommand_dict.begin();
+       pos != m_subcommand_dict.end() && i < maxCount; ++pos, ++i) {
+    buffer.append(" ");
+    buffer.append(pos->first);
+    buffer.append(",");
+  }
+  if (i < m_subcommand_dict.size())
+    buffer.append(" and others");
+  else
+    buffer.pop_back();
+
+  buffer.append(".");
+  return buffer;
+}
+
 void CommandObjectMultiword::GenerateHelpText(Stream &output_stream) {
   // First time through here, generate the help text for the object and push it
   // to the return result object as well

lldb/test/Shell/Commands/command-wrong-subcommand-error-msg.test

@@ -0,0 +1,8 @@
+# UNSUPPORTED: system-windows
+#
+# RUN: %clang_host -g -O0 %S/Inputs/main.c -o %t.out
+# RUN: not %lldb -b -o 'breakpoint foo' %t.out -o exit 2>&1 | FileCheck %s --check-prefix BP-MSG
+# RUN: not %lldb -b -o 'watchpoint set foo' %t.out -o exit 2>&1 | FileCheck %s --check-prefix WP-MSG
+# CHECK: at main.c:2:21
+# BP-MSG: "foo" is not a valid subcommand of "breakpoint". Valid subcommands are: clear, command, delete, disable, enable, and others. Use "help breakpoint" to find out more.
+# WP-MSG: "foo" is not a valid subcommand of "watchpoint set". Valid subcommands are: expression, variable. Use "help watchpoint set" to find out more.