Make X[()] count as a type application (#20568)

Fixes #20563

Author: A5rocks

mypy/typeanal.py

@@ -2533,7 +2533,9 @@ def validate_instance(t: Instance, fail: MsgCallback, empty_tuple_index: bool) -
         arg_count = len(t.args)
         min_tv_count = sum(not tv.has_default() for tv in t.type.defn.type_vars)
         max_tv_count = len(t.type.type_vars)
-        if arg_count and (arg_count < min_tv_count or arg_count > max_tv_count):
+        if (arg_count or empty_tuple_index) and (
+            arg_count < min_tv_count or arg_count > max_tv_count
+        ):
             fail(
                 wrong_type_arg_count(min_tv_count, max_tv_count, str(arg_count), t.type.name),
                 t,

test-data/unit/check-generics.test

@@ -3622,3 +3622,33 @@ t2.foo = [B()]
 t2.foo = [C()]
 t2.foo = [1]  # E: Value of type variable "T" of "foo" of "Test" cannot be "int"
 [builtins fixtures/property.pyi]
+
+[case testMissingTypeArgsInApplication]
+from typing import Generic
+from typing_extensions import Unpack, TypeVarTuple
+
+Ts = TypeVarTuple("Ts")
+
+class CanHaveZero(Generic[Unpack[Ts]]): ...
+
+ok1: CanHaveZero[()]
+ok2: tuple[()]
+
+bad1: list[()] = []  # E: "list" expects 1 type argument, but none given
+[builtins fixtures/tuple.pyi]
+
+[case testMissingTypeArgsInApplicationStrict]
+# flags: --strict
+from typing import Generic
+from typing_extensions import Unpack, TypeVarTuple
+
+Ts = TypeVarTuple("Ts")
+
+class CanHaveZero(Generic[Unpack[Ts]]): ...
+
+ok1: CanHaveZero[()]
+ok2: tuple[()]
+
+bad1: list[()] = []  # E: "list" expects 1 type argument, but none given \
+                     # E: Missing type arguments for generic type "list"
+[builtins fixtures/tuple.pyi]