Fix error message for unrecognized multicharacter short options

CHANGES.rst

@@ -4,6 +4,8 @@ Unreleased
 
 -   Fix handling of ``flag_value`` when ``is_flag=False`` to allow such options to be
     used without an explicit value. :issue:`3084`
+-   Fix error message for unrecognized multicharacter short options (e.g. ``-dbg``)
+    to report the full option name instead of just the first character. :issue:`2779`
 
 Version 8.3.1
 --------------

src/click/parser.py

@@ -490,7 +490,17 @@ def _process_opts(self, arg: str, state: _ParsingState) -> None:
             # short option code and will instead raise the no option
             # error.
             if arg[:2] not in self._opt_prefixes:
-                self._match_short_opt(arg, state)
+                try:
+                    self._match_short_opt(arg, state)
+                except NoSuchOption as e:
+                    # If the short option parser fails on the very first
+                    # character, the original argument is likely a
+                    # multicharacter short option (e.g. ``-dbg``) that
+                    # wasn't found, so report the full original option
+                    # name instead of just the first character.
+                    if e.option_name == f"{arg[0]}{arg[1]}":
+                        raise NoSuchOption(long_opt, ctx=self.ctx) from None
+                    raise
                 return
 
             if not self.ignore_unknown_options:

tests/test_parser.py

@@ -30,3 +30,27 @@ def test_parser_collects_prefixes():
     click.Option("+p", is_flag=True).add_to_parser(parser, ctx)
     click.Option("!e", is_flag=True).add_to_parser(parser, ctx)
     assert parser._opt_prefixes == {"-", "--", "+", "!"}
+
+
+def test_multichar_short_option_error_message():
+    """A multicharacter short option that isn't found should report the
+    full option name in the error, not just the first character. See #2779."""
+
+    @click.command()
+    @click.option("-dbg", is_flag=True)
+    def cli(dbg):
+        pass
+
+    runner = click.testing.CliRunner()
+
+    # Passing the registered option should work.
+    result = runner.invoke(cli, ["-dbg"])
+    assert result.exit_code == 0
+
+    # Passing an unrecognized variant should report the full option name,
+    # not just the first character of the option.
+    result = runner.invoke(cli, ["-dbgwrong"])
+    assert result.exit_code != 0
+    assert "No such option: -dbgwrong" in result.output
+    # The error should NOT be truncated to just the first character "-d".
+    assert result.output.rstrip().endswith("No such option: -dbgwrong")