Clarify that a TypeVarTuple default always has to be unpacked (#1971)

* Clarify that a TypeVarTuple default always has to be unpacked.

The spec says that:
1. The default of a TypeVarTuple can be another TypeVarTuple
2. TypeVarTuples are always unpacked.
Together, these suggest that the default of a TypeVarTuple has to be an
*unpacked* TypeVarTuple, and indeed
[pyright](https://pyright-play.net/?pyrightVersion=1.1.386&code=GYJw9gtgBALgngBwJYDsDmUkQWEMoAqiApgGoCGIBArggDbEA0UAqiguQMYDWAUAQGcAjFAC8hEhSq0GACgDkgofICU-AQCYxEhGUo16xBYI3zmAE2LBy1OjFFK1ggMzaiuqQbmKBzs1EtrW3s2Dh4AbSUAXRUgA)
and
[mypy](https://mypy-play.net/?mypy=latest&python=3.12&gist=b6b01c533c7930e1e520765b773d1fe1)
both agree. This PR makes that clear and adds a corresponding conformance test.

I don't think this needs a discussion + Typing Council approval, since it's
just a clarification of an existing rule, but happy to go through the full
process if anyone disagrees.

Author: rchen152

conformance/results/mypy/generics_defaults.toml

@@ -12,19 +12,19 @@ generics_defaults.py:55: error: Expression is of type "type[AllTheDefaults[int,
 generics_defaults.py:59: error: Expression is of type "type[AllTheDefaults[int, complex, str, int, DefaultBoolT]]", not "type[AllTheDefaults[int, complex, str, int, bool]]"  [assert-type]
 generics_defaults.py:79: error: Expression is of type "type[Class_ParamSpec[DefaultP]]", not "type[Class_ParamSpec[[str, int]]]"  [assert-type]
 generics_defaults.py:94: error: Expression is of type "type[Class_TypeVarTuple[*DefaultTs]]", not "type[Class_TypeVarTuple[str, int]]"  [assert-type]
-generics_defaults.py:104: error: TypeVar default must be a subtype of the bound type  [misc]
-generics_defaults.py:111: error: TypeVar default must be one of the constraint types  [misc]
-generics_defaults.py:151: error: Expression is of type "type[Foo6[*tuple[Any, ...], Any]]", not "type[Foo6[int, str, [float, bool]]]"  [assert-type]
-generics_defaults.py:151: error: Can only replace ParamSpec with a parameter types list or another ParamSpec, got "str"  [misc]
-generics_defaults.py:152: error: Expression is of type "type[Foo6[*tuple[Any, ...], Any]]", not "type[Foo6[int, str, [bytes]]]"  [assert-type]
-generics_defaults.py:152: error: Can only replace ParamSpec with a parameter types list or another ParamSpec, got "str"  [misc]
-generics_defaults.py:166: error: Expression is of type "Callable[[Self], Self]", not "Callable[[Foo7[int]], Foo7[int]]"  [assert-type]
-generics_defaults.py:167: error: Expression is of type "Any", not "int"  [assert-type]
-generics_defaults.py:167: error: Access to generic instance variables via class is ambiguous  [misc]
+generics_defaults.py:107: error: TypeVar default must be a subtype of the bound type  [misc]
+generics_defaults.py:114: error: TypeVar default must be one of the constraint types  [misc]
+generics_defaults.py:154: error: Expression is of type "type[Foo6[*tuple[Any, ...], Any]]", not "type[Foo6[int, str, [float, bool]]]"  [assert-type]
+generics_defaults.py:154: error: Can only replace ParamSpec with a parameter types list or another ParamSpec, got "str"  [misc]
+generics_defaults.py:155: error: Expression is of type "type[Foo6[*tuple[Any, ...], Any]]", not "type[Foo6[int, str, [bytes]]]"  [assert-type]
+generics_defaults.py:155: error: Can only replace ParamSpec with a parameter types list or another ParamSpec, got "str"  [misc]
+generics_defaults.py:169: error: Expression is of type "Callable[[Self], Self]", not "Callable[[Foo7[int]], Foo7[int]]"  [assert-type]
+generics_defaults.py:170: error: Expression is of type "Any", not "int"  [assert-type]
+generics_defaults.py:170: error: Access to generic instance variables via class is ambiguous  [misc]
 """
 conformance_automated = "Fail"
 errors_diff = """
-Line 138: Expected 1 errors
+Line 141: Expected 1 errors
 Line 30: Unexpected errors ['generics_defaults.py:30: error: Expression is of type "type[NoNonDefaults[DefaultStrT, DefaultIntT]]", not "type[NoNonDefaults[str, int]]"  [assert-type]']
 Line 31: Unexpected errors ['generics_defaults.py:31: error: Expression is of type "type[NoNonDefaults[str, DefaultIntT]]", not "type[NoNonDefaults[str, int]]"  [assert-type]']
 Line 38: Unexpected errors ['generics_defaults.py:38: error: Expression is of type "type[OneDefault[float, DefaultBoolT]]", not "type[OneDefault[float, bool]]"  [assert-type]']
@@ -35,8 +35,8 @@ Line 55: Unexpected errors ['generics_defaults.py:55: error: Expression is of ty
 Line 59: Unexpected errors ['generics_defaults.py:59: error: Expression is of type "type[AllTheDefaults[int, complex, str, int, DefaultBoolT]]", not "type[AllTheDefaults[int, complex, str, int, bool]]"  [assert-type]']
 Line 79: Unexpected errors ['generics_defaults.py:79: error: Expression is of type "type[Class_ParamSpec[DefaultP]]", not "type[Class_ParamSpec[[str, int]]]"  [assert-type]']
 Line 94: Unexpected errors ['generics_defaults.py:94: error: Expression is of type "type[Class_TypeVarTuple[*DefaultTs]]", not "type[Class_TypeVarTuple[str, int]]"  [assert-type]']
-Line 151: Unexpected errors ['generics_defaults.py:151: error: Expression is of type "type[Foo6[*tuple[Any, ...], Any]]", not "type[Foo6[int, str, [float, bool]]]"  [assert-type]', 'generics_defaults.py:151: error: Can only replace ParamSpec with a parameter types list or another ParamSpec, got "str"  [misc]']
-Line 152: Unexpected errors ['generics_defaults.py:152: error: Expression is of type "type[Foo6[*tuple[Any, ...], Any]]", not "type[Foo6[int, str, [bytes]]]"  [assert-type]', 'generics_defaults.py:152: error: Can only replace ParamSpec with a parameter types list or another ParamSpec, got "str"  [misc]']
-Line 166: Unexpected errors ['generics_defaults.py:166: error: Expression is of type "Callable[[Self], Self]", not "Callable[[Foo7[int]], Foo7[int]]"  [assert-type]']
-Line 167: Unexpected errors ['generics_defaults.py:167: error: Expression is of type "Any", not "int"  [assert-type]', 'generics_defaults.py:167: error: Access to generic instance variables via class is ambiguous  [misc]']
+Line 154: Unexpected errors ['generics_defaults.py:154: error: Expression is of type "type[Foo6[*tuple[Any, ...], Any]]", not "type[Foo6[int, str, [float, bool]]]"  [assert-type]', 'generics_defaults.py:154: error: Can only replace ParamSpec with a parameter types list or another ParamSpec, got "str"  [misc]']
+Line 155: Unexpected errors ['generics_defaults.py:155: error: Expression is of type "type[Foo6[*tuple[Any, ...], Any]]", not "type[Foo6[int, str, [bytes]]]"  [assert-type]', 'generics_defaults.py:155: error: Can only replace ParamSpec with a parameter types list or another ParamSpec, got "str"  [misc]']
+Line 169: Unexpected errors ['generics_defaults.py:169: error: Expression is of type "Callable[[Self], Self]", not "Callable[[Foo7[int]], Foo7[int]]"  [assert-type]']
+Line 170: Unexpected errors ['generics_defaults.py:170: error: Expression is of type "Any", not "int"  [assert-type]', 'generics_defaults.py:170: error: Access to generic instance variables via class is ambiguous  [misc]']
 """

conformance/results/pyre/generics_defaults.toml

@@ -50,26 +50,27 @@ generics_defaults.py:95:34 Invalid type parameters [24]: Non-generic type `Class
 generics_defaults.py:96:0 Assert type [70]: Expected `Class_TypeVarTuple[]` but got `typing.Any`.
 generics_defaults.py:96:12 Undefined attribute [16]: `Class_TypeVarTuple` has no attribute `__getitem__`.
 generics_defaults.py:96:45 Invalid type parameters [24]: Non-generic type `Class_TypeVarTuple` cannot take parameters.
-generics_defaults.py:124:13 Undefined or invalid type [11]: Annotation `T4` is not defined as a type.
-generics_defaults.py:127:0 Assert type [70]: Expected `int` but got `unknown`.
-generics_defaults.py:138:11 Invalid type variable [34]: The current class isn't generic with respect to the type variable `Ts`.
-generics_defaults.py:138:11 Undefined or invalid type [11]: Annotation `T5` is not defined as a type.
-generics_defaults.py:148:11 Invalid type variable [34]: The current class isn't generic with respect to the type variable `Ts`.
-generics_defaults.py:148:11 Undefined or invalid type [11]: Annotation `P` is not defined as a type.
-generics_defaults.py:151:0 Assert type [70]: Expected `Type[Foo6[]]` but got `typing.Any`.
-generics_defaults.py:151:12 Undefined attribute [16]: `Foo6` has no attribute `__getitem__`.
-generics_defaults.py:151:28 Invalid type parameters [24]: Non-generic type `Foo6` cannot take parameters.
-generics_defaults.py:152:0 Assert type [70]: Expected `Type[Foo6[]]` but got `typing.Any`.
-generics_defaults.py:152:37 Invalid type parameters [24]: Non-generic type `Foo6` cannot take parameters.
-generics_defaults.py:166:0 Assert type [70]: Expected `typing.Callable[[Foo7[]], Foo7[]]` but got `typing.Callable(Foo7.meth)[[Variable[_Self_generics_defaults_Foo7__ (bound to Foo7)]], Variable[_Self_generics_defaults_Foo7__ (bound to Foo7)]]`.
-generics_defaults.py:166:23 Invalid type parameters [24]: Non-generic type `Foo7` cannot take parameters.
-generics_defaults.py:167:0 Assert type [70]: Expected `int` but got `unknown`.
+generics_defaults.py:99:67 Incompatible parameter type [6]: In call `typing.GenericMeta.__getitem__`, for 1st positional argument, expected `Type[Variable[_T]]` but got `TypeVarTuple`.
+generics_defaults.py:127:13 Undefined or invalid type [11]: Annotation `T4` is not defined as a type.
+generics_defaults.py:130:0 Assert type [70]: Expected `int` but got `unknown`.
+generics_defaults.py:141:11 Invalid type variable [34]: The current class isn't generic with respect to the type variable `Ts`.
+generics_defaults.py:141:11 Undefined or invalid type [11]: Annotation `T5` is not defined as a type.
+generics_defaults.py:151:11 Invalid type variable [34]: The current class isn't generic with respect to the type variable `Ts`.
+generics_defaults.py:151:11 Undefined or invalid type [11]: Annotation `P` is not defined as a type.
+generics_defaults.py:154:0 Assert type [70]: Expected `Type[Foo6[]]` but got `typing.Any`.
+generics_defaults.py:154:12 Undefined attribute [16]: `Foo6` has no attribute `__getitem__`.
+generics_defaults.py:154:28 Invalid type parameters [24]: Non-generic type `Foo6` cannot take parameters.
+generics_defaults.py:155:0 Assert type [70]: Expected `Type[Foo6[]]` but got `typing.Any`.
+generics_defaults.py:155:37 Invalid type parameters [24]: Non-generic type `Foo6` cannot take parameters.
+generics_defaults.py:169:0 Assert type [70]: Expected `typing.Callable[[Foo7[]], Foo7[]]` but got `typing.Callable(Foo7.meth)[[Variable[_Self_generics_defaults_Foo7__ (bound to Foo7)]], Variable[_Self_generics_defaults_Foo7__ (bound to Foo7)]]`.
+generics_defaults.py:169:23 Invalid type parameters [24]: Non-generic type `Foo7` cannot take parameters.
+generics_defaults.py:170:0 Assert type [70]: Expected `int` but got `unknown`.
 """
 conformance_automated = "Fail"
 errors_diff = """
 Line 50: Expected 1 errors
-Line 104: Expected 1 errors
-Line 111: Expected 1 errors
+Line 107: Expected 1 errors
+Line 114: Expected 1 errors
 Line 27: Unexpected errors ['generics_defaults.py:27:20 Undefined or invalid type [11]: Annotation `DefaultIntT` is not defined as a type.']
 Line 30: Unexpected errors ['generics_defaults.py:30:0 Assert type [70]: Expected `Type[NoNonDefaults[]]` but got `Type[NoNonDefaults]`.', 'generics_defaults.py:30:27 Invalid type parameters [24]: Non-generic type `NoNonDefaults` cannot take parameters.']
 Line 31: Unexpected errors ['generics_defaults.py:31:0 Assert type [70]: Expected `Type[NoNonDefaults[]]` but got `typing.Any`.', 'generics_defaults.py:31:12 Undefined attribute [16]: `NoNonDefaults` has no attribute `__getitem__`.', 'generics_defaults.py:31:32 Invalid type parameters [24]: Non-generic type `NoNonDefaults` cannot take parameters.']
@@ -97,11 +98,12 @@ Line 91: Unexpected errors ['generics_defaults.py:91:25 Undefined or invalid typ
 Line 94: Unexpected errors ['generics_defaults.py:94:0 Assert type [70]: Expected `Type[Class_TypeVarTuple[]]` but got `Type[Class_TypeVarTuple]`.', 'generics_defaults.py:94:32 Invalid type parameters [24]: Non-generic type `Class_TypeVarTuple` cannot take parameters.']
 Line 95: Unexpected errors ['generics_defaults.py:95:0 Assert type [70]: Expected `Class_TypeVarTuple[]` but got `Class_TypeVarTuple`.', 'generics_defaults.py:95:34 Invalid type parameters [24]: Non-generic type `Class_TypeVarTuple` cannot take parameters.']
 Line 96: Unexpected errors ['generics_defaults.py:96:0 Assert type [70]: Expected `Class_TypeVarTuple[]` but got `typing.Any`.', 'generics_defaults.py:96:12 Undefined attribute [16]: `Class_TypeVarTuple` has no attribute `__getitem__`.', 'generics_defaults.py:96:45 Invalid type parameters [24]: Non-generic type `Class_TypeVarTuple` cannot take parameters.']
-Line 124: Unexpected errors ['generics_defaults.py:124:13 Undefined or invalid type [11]: Annotation `T4` is not defined as a type.']
-Line 127: Unexpected errors ['generics_defaults.py:127:0 Assert type [70]: Expected `int` but got `unknown`.']
-Line 148: Unexpected errors ["generics_defaults.py:148:11 Invalid type variable [34]: The current class isn't generic with respect to the type variable `Ts`.", 'generics_defaults.py:148:11 Undefined or invalid type [11]: Annotation `P` is not defined as a type.']
-Line 151: Unexpected errors ['generics_defaults.py:151:0 Assert type [70]: Expected `Type[Foo6[]]` but got `typing.Any`.', 'generics_defaults.py:151:12 Undefined attribute [16]: `Foo6` has no attribute `__getitem__`.', 'generics_defaults.py:151:28 Invalid type parameters [24]: Non-generic type `Foo6` cannot take parameters.']
-Line 152: Unexpected errors ['generics_defaults.py:152:0 Assert type [70]: Expected `Type[Foo6[]]` but got `typing.Any`.', 'generics_defaults.py:152:37 Invalid type parameters [24]: Non-generic type `Foo6` cannot take parameters.']
-Line 166: Unexpected errors ['generics_defaults.py:166:0 Assert type [70]: Expected `typing.Callable[[Foo7[]], Foo7[]]` but got `typing.Callable(Foo7.meth)[[Variable[_Self_generics_defaults_Foo7__ (bound to Foo7)]], Variable[_Self_generics_defaults_Foo7__ (bound to Foo7)]]`.', 'generics_defaults.py:166:23 Invalid type parameters [24]: Non-generic type `Foo7` cannot take parameters.']
-Line 167: Unexpected errors ['generics_defaults.py:167:0 Assert type [70]: Expected `int` but got `unknown`.']
+Line 99: Unexpected errors ['generics_defaults.py:99:67 Incompatible parameter type [6]: In call `typing.GenericMeta.__getitem__`, for 1st positional argument, expected `Type[Variable[_T]]` but got `TypeVarTuple`.']
+Line 127: Unexpected errors ['generics_defaults.py:127:13 Undefined or invalid type [11]: Annotation `T4` is not defined as a type.']
+Line 130: Unexpected errors ['generics_defaults.py:130:0 Assert type [70]: Expected `int` but got `unknown`.']
+Line 151: Unexpected errors ["generics_defaults.py:151:11 Invalid type variable [34]: The current class isn't generic with respect to the type variable `Ts`.", 'generics_defaults.py:151:11 Undefined or invalid type [11]: Annotation `P` is not defined as a type.']
+Line 154: Unexpected errors ['generics_defaults.py:154:0 Assert type [70]: Expected `Type[Foo6[]]` but got `typing.Any`.', 'generics_defaults.py:154:12 Undefined attribute [16]: `Foo6` has no attribute `__getitem__`.', 'generics_defaults.py:154:28 Invalid type parameters [24]: Non-generic type `Foo6` cannot take parameters.']
+Line 155: Unexpected errors ['generics_defaults.py:155:0 Assert type [70]: Expected `Type[Foo6[]]` but got `typing.Any`.', 'generics_defaults.py:155:37 Invalid type parameters [24]: Non-generic type `Foo6` cannot take parameters.']
+Line 169: Unexpected errors ['generics_defaults.py:169:0 Assert type [70]: Expected `typing.Callable[[Foo7[]], Foo7[]]` but got `typing.Callable(Foo7.meth)[[Variable[_Self_generics_defaults_Foo7__ (bound to Foo7)]], Variable[_Self_generics_defaults_Foo7__ (bound to Foo7)]]`.', 'generics_defaults.py:169:23 Invalid type parameters [24]: Non-generic type `Foo7` cannot take parameters.']
+Line 170: Unexpected errors ['generics_defaults.py:170:0 Assert type [70]: Expected `int` but got `unknown`.']
 """

conformance/results/pyright/generics_defaults.toml

@@ -2,10 +2,10 @@ conformant = "Pass"
 output = """
 generics_defaults.py:24:7 - error: "T" cannot appear after "DefaultStrT" in type parameter list because it has no default type (reportGeneralTypeIssues)
 generics_defaults.py:50:16 - error: Too few type arguments provided for "AllTheDefaults"; expected 2 but received 1 (reportInvalidTypeArguments)
-generics_defaults.py:104:51 - error: TypeVar default type must be a subtype of the bound type (reportGeneralTypeIssues)
-generics_defaults.py:111:52 - error: TypeVar default type must be one of the constrained types (reportGeneralTypeIssues)
-generics_defaults.py:138:7 - error: TypeVar "T5" has a default value and cannot follow TypeVarTuple "Ts" (reportGeneralTypeIssues)
-generics_defaults.py:167:18 - error: Access to generic instance variable through class is ambiguous (reportGeneralTypeIssues)
+generics_defaults.py:107:51 - error: TypeVar default type must be a subtype of the bound type (reportGeneralTypeIssues)
+generics_defaults.py:114:52 - error: TypeVar default type must be one of the constrained types (reportGeneralTypeIssues)
+generics_defaults.py:141:7 - error: TypeVar "T5" has a default value and cannot follow TypeVarTuple "Ts" (reportGeneralTypeIssues)
+generics_defaults.py:170:18 - error: Access to generic instance variable through class is ambiguous (reportGeneralTypeIssues)
 """
 conformance_automated = "Pass"
 errors_diff = """