shell completion prioritizes option values over new options

Allow option values that start with an option prefix to
complete rather than treating them like new options.

Don't treat count options as needing a value to complete.

7 files changed, 134 insertions, 9 deletions

diff --git a/CHANGES.rst b/CHANGES.rst
index 91b9ec1..191937d 100644
--- a/CHANGES.rst
+++ b/CHANGES.rst
@@ -50,6 +50,8 @@ Version 8.1.0
     value. :issue:`2124`
 -   ``to_info_dict`` will not fail if a ``ParamType`` doesn't define a
     ``name``. :issue:`2168`
+-   Shell completion prioritizes option values with option prefixes over
+    new options. :issue:`2040`
 
 
 Version 8.0.4
diff --git a/src/click/core.py b/src/click/core.py
index 263a5ff..939ad81 100644
--- a/src/click/core.py
+++ b/src/click/core.py
@@ -292,6 +292,8 @@ class Context:
         #: must be never propagated to another arguments.  This is used
         #: to implement nested parsing.
         self.protected_args: t.List[str] = []
+        #: the collected prefixes of the command's options.
+        self._opt_prefixes: t.Set[str] = set(parent._opt_prefixes) if parent else set()
 
         if obj is None and parent is not None:
             obj = parent.obj
@@ -1385,6 +1387,7 @@ class Command(BaseCommand):
             )
 
         ctx.args = args
+        ctx._opt_prefixes.update(parser._opt_prefixes)
         return args
 
     def invoke(self, ctx: Context) -> t.Any:
diff --git a/src/click/shell_completion.py b/src/click/shell_completion.py
index 1e9df6f..c17a8e6 100644
--- a/src/click/shell_completion.py
+++ b/src/click/shell_completion.py
@@ -448,17 +448,16 @@ def _is_incomplete_argument(ctx: Context, param: Parameter) -> bool:
     )
 
 
-def _start_of_option(value: str) -> bool:
+def _start_of_option(ctx: Context, value: str) -> bool:
     """Check if the value looks like the start of an option."""
     if not value:
         return False
 
     c = value[0]
-    # Allow "/" since that starts a path.
-    return not c.isalnum() and c != "/"
+    return c in ctx._opt_prefixes
 
 
-def _is_incomplete_option(args: t.List[str], param: Parameter) -> bool:
+def _is_incomplete_option(ctx: Context, args: t.List[str], param: Parameter) -> bool:
     """Determine if the given parameter is an option that needs a value.
 
     :param args: List of complete args before the incomplete value.
@@ -467,7 +466,7 @@ def _is_incomplete_option(args: t.List[str], param: Parameter) -> bool:
     if not isinstance(param, Option):
         return False
 
-    if param.is_flag:
+    if param.is_flag or param.count:
         return False
 
     last_option = None
@@ -476,7 +475,7 @@ def _is_incomplete_option(args: t.List[str], param: Parameter) -> bool:
         if index + 1 > param.nargs:
             break
 
-        if _start_of_option(arg):
+        if _start_of_option(ctx, arg):
             last_option = arg
 
     return last_option is not None and last_option in param.opts
@@ -551,7 +550,7 @@ def _resolve_incomplete(
     # split and discard the "=" to make completion easier.
     if incomplete == "=":
         incomplete = ""
-    elif "=" in incomplete and _start_of_option(incomplete):
+    elif "=" in incomplete and _start_of_option(ctx, incomplete):
         name, _, incomplete = incomplete.partition("=")
         args.append(name)
 
@@ -559,7 +558,7 @@ def _resolve_incomplete(
     # even if they start with the option character. If it hasn't been
     # given and the incomplete arg looks like an option, the current
     # command will provide option name completions.
-    if "--" not in args and _start_of_option(incomplete):
+    if "--" not in args and _start_of_option(ctx, incomplete):
         return ctx.command, incomplete
 
     params = ctx.command.get_params(ctx)
@@ -567,7 +566,7 @@ def _resolve_incomplete(
     # If the last complete arg is an option name with an incomplete
     # value, the option will provide value completions.
     for param in params:
-        if _is_incomplete_option(args, param):
+        if _is_incomplete_option(ctx, args, param):
             return param, incomplete
 
     # It's not an option name or value. The first argument without a
diff --git a/tests/test_commands.py b/tests/test_commands.py
index bf6a5db..3a0d4b9 100644
--- a/tests/test_commands.py
+++ b/tests/test_commands.py
@@ -352,3 +352,62 @@ def test_deprecated_in_invocation(runner):
 
     result = runner.invoke(deprecated_cmd)
     assert "DeprecationWarning:" in result.output
+
+
+def test_command_parse_args_collects_option_prefixes():
+    @click.command()
+    @click.option("+p", is_flag=True)
+    @click.option("!e", is_flag=True)
+    def test(p, e):
+        pass
+
+    ctx = click.Context(test)
+    test.parse_args(ctx, [])
+
+    assert ctx._opt_prefixes == {"-", "--", "+", "!"}
+
+
+def test_group_parse_args_collects_base_option_prefixes():
+    @click.group()
+    @click.option("~t", is_flag=True)
+    def group(t):
+        pass
+
+    @group.command()
+    @click.option("+p", is_flag=True)
+    def command1(p):
+        pass
+
+    @group.command()
+    @click.option("!e", is_flag=True)
+    def command2(e):
+        pass
+
+    ctx = click.Context(group)
+    group.parse_args(ctx, ["command1", "+p"])
+
+    assert ctx._opt_prefixes == {"-", "--", "~"}
+
+
+def test_group_invoke_collects_used_option_prefixes(runner):
+    opt_prefixes = set()
+
+    @click.group()
+    @click.option("~t", is_flag=True)
+    def group(t):
+        pass
+
+    @group.command()
+    @click.option("+p", is_flag=True)
+    @click.pass_context
+    def command1(ctx, p):
+        nonlocal opt_prefixes
+        opt_prefixes = ctx._opt_prefixes
+
+    @group.command()
+    @click.option("!e", is_flag=True)
+    def command2(e):
+        pass
+
+    runner.invoke(group, ["command1"])
+    assert opt_prefixes == {"-", "--", "~", "+"}
diff --git a/tests/test_context.py b/tests/test_context.py
index 98f0835..df8b497 100644
--- a/tests/test_context.py
+++ b/tests/test_context.py
@@ -365,3 +365,11 @@ def test_parameter_source(runner, option_args, invoke_args, expect):
 
     rv = runner.invoke(cli, standalone_mode=False, **invoke_args)
     assert rv.return_value == expect
+
+
+def test_propagate_opt_prefixes():
+    parent = click.Context(click.Command("test"))
+    parent._opt_prefixes = {"-", "--", "!"}
+    ctx = click.Context(click.Command("test2"), parent=parent)
+
+    assert ctx._opt_prefixes == {"-", "--", "!"}
diff --git a/tests/test_parser.py b/tests/test_parser.py
index 964f9c8..f694916 100644
--- a/tests/test_parser.py
+++ b/tests/test_parser.py
@@ -1,5 +1,7 @@
 import pytest
 
+import click
+from click.parser import OptionParser
 from click.parser import split_arg_string
 
 
@@ -15,3 +17,16 @@ from click.parser import split_arg_string
 )
 def test_split_arg_string(value, expect):
     assert split_arg_string(value) == expect
+
+
+def test_parser_default_prefixes():
+    parser = OptionParser()
+    assert parser._opt_prefixes == {"-", "--"}
+
+
+def test_parser_collects_prefixes():
+    ctx = click.Context(click.Command("test"))
+    parser = OptionParser(ctx)
+    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 == {"-", "--", "+", "!"}
diff --git a/tests/test_shell_completion.py b/tests/test_shell_completion.py
index 08fa057..57729f5 100644
--- a/tests/test_shell_completion.py
+++ b/tests/test_shell_completion.py
@@ -110,6 +110,45 @@ def test_type_choice():
     assert _get_words(cli, ["-c"], "a2") == ["a2"]
 
 
+def test_choice_special_characters():
+    cli = Command("cli", params=[Option(["-c"], type=Choice(["!1", "!2", "+3"]))])
+    assert _get_words(cli, ["-c"], "") == ["!1", "!2", "+3"]
+    assert _get_words(cli, ["-c"], "!") == ["!1", "!2"]
+    assert _get_words(cli, ["-c"], "!2") == ["!2"]
+
+
+def test_choice_conflicting_prefix():
+    cli = Command(
+        "cli",
+        params=[
+            Option(["-c"], type=Choice(["!1", "!2", "+3"])),
+            Option(["+p"], is_flag=True),
+        ],
+    )
+    assert _get_words(cli, ["-c"], "") == ["!1", "!2", "+3"]
+    assert _get_words(cli, ["-c"], "+") == ["+p"]
+
+
+def test_option_count():
+    cli = Command("cli", params=[Option(["-c"], count=True)])
+    assert _get_words(cli, ["-c"], "") == []
+    assert _get_words(cli, ["-c"], "-") == ["--help"]
+
+
+def test_option_optional():
+    cli = Command(
+        "cli",
+        add_help_option=False,
+        params=[
+            Option(["--name"], is_flag=False, flag_value="value"),
+            Option(["--flag"], is_flag=True),
+        ],
+    )
+    assert _get_words(cli, ["--name"], "") == []
+    assert _get_words(cli, ["--name"], "-") == ["--flag"]
+    assert _get_words(cli, ["--name", "--flag"], "-") == []
+
+
 @pytest.mark.parametrize(
     ("type", "expect"),
     [(File(), "file"), (Path(), "file"), (Path(file_okay=False), "dir")],