Reject bound covariant type variable in List argument

python · Aug 6, 2020 · 85d5ba9 · 85d5ba9
1 parent c5814e5
commit 85d5ba9
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diff --git a/mypy/checker.py b/mypy/checker.py
@@ -947,6 +947,14 @@ def check_func_def(self, defn: FuncItem, typ: CallableType, name: Optional[str])
                             if ctx.line < 0:
                                 ctx = typ
                             self.fail(message_registry.FUNCTION_PARAMETER_CANNOT_BE_COVARIANT, ctx)
+                    elif is_named_instance(arg_type, 'builtins.list'):
+                        arg_typ = get_proper_type(arg_type)
+                        if isinstance(arg_typ, Instance):
+                            item_type = self.iterable_item_type(arg_typ)
+                            if (isinstance(item_type, TypeVarType)
+                                    and item_type.variance == COVARIANT):
+                                message = "Cannot use a covariant type variable as a parameter"
+                                self.fail(message, arg_type)
                     if typ.arg_kinds[i] == nodes.ARG_STAR:
                         # builtins.tuple[T] is typing.Tuple[T, ...]
                         arg_type = self.named_generic_type('builtins.tuple',

diff --git a/test-data/unit/check-functions.test b/test-data/unit/check-functions.test
@@ -2066,6 +2066,18 @@ class A(Generic[t]):
 [out]
 main:5: error: Cannot use a covariant type variable as a parameter
 
+[case testRejectListCovariantArgument]
+from typing import TypeVar, List, Generic
+
+t = TypeVar('t', covariant=True)
+class A(Generic[t]):
+    def foo(self, x: List[t]) -> None:
+        return None
+[builtins fixtures/bool.pyi]
+[builtins fixtures/list.pyi]
+[out]
+main:5: error: Cannot use a covariant type variable as a parameter
+
 [case testRejectCovariantArgumentSplitLine]
 from typing import TypeVar, Generic