stubtest: allow ellipsis as default argument (#12838)

Resolves #12819

Author: hauntsaninja

mypy/stubtest.py

@@ -419,6 +419,8 @@ def _verify_arg_default_value(
                 and stub_type is not None
                 # Avoid false positives for marker objects
                 and type(runtime_arg.default) != object
+                # And ellipsis
+                and runtime_arg.default is not ...
                 and not is_subtype_helper(runtime_type, stub_type)
             ):
                 yield (

mypy/test/teststubtest.py

@@ -43,6 +43,7 @@ def __getitem__(self, typeargs: Any) -> object: ...
 
 Callable: _SpecialForm = ...
 Generic: _SpecialForm = ...
+Protocol: _SpecialForm = ...
 
 class TypeVar:
     def __init__(self, name, covariant: bool = ..., contravariant: bool = ...) -> None: ...
@@ -1020,6 +1021,27 @@ class _Options(TypedDict):
             error="opt3",
         )
 
+    @collect_cases
+    def test_protocol(self) -> Iterator[Case]:
+        if sys.version_info < (3, 7):
+            return
+        yield Case(
+            stub="""
+            from typing_extensions import Protocol
+
+            class X(Protocol):
+                def foo(self, x: int, y: bytes = ...) -> str: ...
+            """,
+            runtime="""
+            from typing_extensions import Protocol
+
+            class X(Protocol):
+                def foo(self, x: int, y: bytes = ...) -> str: ...
+            """,
+            # TODO: this should not be an error, #12820
+            error="X.__init__"
+        )
+
 
 def remove_color_code(s: str) -> str:
     return re.sub("\\x1b.*?m", "", s)  # this works!