Updated spec and conformance test to reflect that a type variable tha… (

#1576)

* Updated spec and conformance test to reflect that a type variable that is defined as covariant or contravariant should not generate an error when bound to a generic function or type alias. Variance has no meaning in this context, so it should be ignored. This brings the spec in alignment with the behavior of the major type checkers.

* Added comment as per PR suggestion.

* Reran tests.

* Fixed a botched merge that undid a previous PR.

conformance/results/mypy/generics_variance.toml

@@ -1,14 +1,13 @@
 conformant = "Partial"
 notes = """
-Does not reject a function-scoped TypeVar that is marked as covariant or contravariant.
 Does not reject use of class-scoped TypeVar used in a base class when variance is incompatible.
 """
 output = """
 generics_variance.py:14: error: TypeVar cannot be both covariant and contravariant  [misc]
-generics_variance.py:78: error: Variance of TypeVar "T_co" incompatible with variance in parent type  [type-var]
-generics_variance.py:82: error: Variance of TypeVar "T_contra" incompatible with variance in parent type  [type-var]
-generics_variance.py:94: error: Variance of TypeVar "T_contra" incompatible with variance in parent type  [type-var]
-generics_variance.py:106: error: Variance of TypeVar "T_co" incompatible with variance in parent type  [type-var]
-generics_variance.py:126: error: Variance of TypeVar "T_co" incompatible with variance in parent type  [type-var]
-generics_variance.py:132: error: Variance of TypeVar "T_contra" incompatible with variance in parent type  [type-var]
+generics_variance.py:77: error: Variance of TypeVar "T_co" incompatible with variance in parent type  [type-var]
+generics_variance.py:81: error: Variance of TypeVar "T_contra" incompatible with variance in parent type  [type-var]
+generics_variance.py:93: error: Variance of TypeVar "T_contra" incompatible with variance in parent type  [type-var]
+generics_variance.py:105: error: Variance of TypeVar "T_co" incompatible with variance in parent type  [type-var]
+generics_variance.py:125: error: Variance of TypeVar "T_co" incompatible with variance in parent type  [type-var]
+generics_variance.py:131: error: Variance of TypeVar "T_contra" incompatible with variance in parent type  [type-var]
 """

conformance/results/pyre/generics_variance.toml

@@ -1,14 +1,13 @@
 conformant = "Partial"
 notes = """
 Does not reject a TypeVar that is defined as both covariant and contravariant.
-Does not reject a function-scoped TypeVar that is marked as covariant or contravariant.
 Does not reject use of class-scoped TypeVar used in a base class when variance is incompatible.
 """
 output = """
-generics_variance.py:78:0 Invalid type variance [46]: The type variable `Variable[T_co](covariant)` is incompatible with parent class type variable `Variable[T]` because subclasses cannot use more permissive type variables than their superclasses.
-generics_variance.py:82:0 Invalid type variance [46]: The type variable `Variable[T_contra](contravariant)` is incompatible with parent class type variable `Variable[T]` because subclasses cannot use more permissive type variables than their superclasses.
-generics_variance.py:94:0 Invalid type variance [46]: The type variable `Variable[T_contra](contravariant)` is incompatible with parent class type variable `Variable[T_co](covariant)` because subclasses cannot use more permissive type variables than their superclasses.
-generics_variance.py:106:0 Invalid type variance [46]: The type variable `Variable[T_co](covariant)` is incompatible with parent class type variable `Variable[T_contra](contravariant)` because subclasses cannot use more permissive type variables than their superclasses.
-generics_variance.py:126:0 Invalid type variance [46]: The type variable `Variable[T_co](covariant)` is incompatible with parent class type variable `Variable[T_contra](contravariant)` because subclasses cannot use more permissive type variables than their superclasses.
-generics_variance.py:132:0 Invalid type variance [46]: The type variable `Variable[T_contra](contravariant)` is incompatible with parent class type variable `Variable[T_co](covariant)` because subclasses cannot use more permissive type variables than their superclasses.
+generics_variance.py:77:0 Invalid type variance [46]: The type variable `Variable[T_co](covariant)` is incompatible with parent class type variable `Variable[T]` because subclasses cannot use more permissive type variables than their superclasses.
+generics_variance.py:81:0 Invalid type variance [46]: The type variable `Variable[T_contra](contravariant)` is incompatible with parent class type variable `Variable[T]` because subclasses cannot use more permissive type variables than their superclasses.
+generics_variance.py:93:0 Invalid type variance [46]: The type variable `Variable[T_contra](contravariant)` is incompatible with parent class type variable `Variable[T_co](covariant)` because subclasses cannot use more permissive type variables than their superclasses.
+generics_variance.py:105:0 Invalid type variance [46]: The type variable `Variable[T_co](covariant)` is incompatible with parent class type variable `Variable[T_contra](contravariant)` because subclasses cannot use more permissive type variables than their superclasses.
+generics_variance.py:125:0 Invalid type variance [46]: The type variable `Variable[T_co](covariant)` is incompatible with parent class type variable `Variable[T_contra](contravariant)` because subclasses cannot use more permissive type variables than their superclasses.
+generics_variance.py:131:0 Invalid type variance [46]: The type variable `Variable[T_contra](contravariant)` is incompatible with parent class type variable `Variable[T_co](covariant)` because subclasses cannot use more permissive type variables than their superclasses.
 """

conformance/results/pyright/generics_variance.toml

@@ -1,31 +1,28 @@
-conformant = "Partial"
-notes = """
-Does not reject a function-scoped TypeVar that is marked as covariant or contravariant.
-"""
+conformant = "Pass"
 output = """
 generics_variance.py:14:50 - error: TypeVar cannot be both covariant and contravariant
-generics_variance.py:78:18 - error: Type "T_co@Class1" cannot be assigned to type variable "T@Inv"
+generics_variance.py:77:18 - error: Type "T_co@Class1" cannot be assigned to type variable "T@Inv"
   Variance of type argument "T_co@Class1" is incompatible with base class "Inv" (reportGeneralTypeIssues)
-generics_variance.py:82:18 - error: Type "T_contra@Class2" cannot be assigned to type variable "T@Inv"
+generics_variance.py:81:18 - error: Type "T_contra@Class2" cannot be assigned to type variable "T@Inv"
   Variance of type argument "T_contra@Class2" is incompatible with base class "Inv" (reportGeneralTypeIssues)
-generics_variance.py:94:20 - error: Type "T_contra@Co_Child3" cannot be assigned to type variable "T_co@Co"
+generics_variance.py:93:20 - error: Type "T_contra@Co_Child3" cannot be assigned to type variable "T_co@Co"
   Variance of type argument "T_contra@Co_Child3" is incompatible with base class "Co" (reportGeneralTypeIssues)
-generics_variance.py:106:28 - error: Type "T_co@Contra_Child3" cannot be assigned to type variable "T_contra@Contra"
+generics_variance.py:105:28 - error: Type "T_co@Contra_Child3" cannot be assigned to type variable "T_contra@Contra"
   Variance of type argument "T_co@Contra_Child3" is incompatible with base class "Contra" (reportGeneralTypeIssues)
-generics_variance.py:114:28 - error: Type "Co[T_co@Contra_Child5]" cannot be assigned to type variable "T_contra@Contra"
+generics_variance.py:113:28 - error: Type "Co[T_co@Contra_Child5]" cannot be assigned to type variable "T_contra@Contra"
   Variance of type argument "Co[T_co@Contra_Child5]" is incompatible with base class "Contra" (reportGeneralTypeIssues)
-generics_variance.py:127:20 - error: Type "T_co@CoContra_Child2" cannot be assigned to type variable "T_contra@CoContra"
+generics_variance.py:126:20 - error: Type "T_co@CoContra_Child2" cannot be assigned to type variable "T_contra@CoContra"
   Variance of type argument "T_co@CoContra_Child2" is incompatible with base class "CoContra" (reportGeneralTypeIssues)
-generics_variance.py:133:14 - error: Type "T_contra@CoContra_Child3" cannot be assigned to type variable "T_co@CoContra"
+generics_variance.py:132:14 - error: Type "T_contra@CoContra_Child3" cannot be assigned to type variable "T_co@CoContra"
   Variance of type argument "T_contra@CoContra_Child3" is incompatible with base class "CoContra" (reportGeneralTypeIssues)
-generics_variance.py:143:24 - error: Type "Co[T_co@CoContra_Child5]" cannot be assigned to type variable "T_contra@CoContra"
+generics_variance.py:142:24 - error: Type "Co[T_co@CoContra_Child5]" cannot be assigned to type variable "T_contra@CoContra"
   Variance of type argument "Co[T_co@CoContra_Child5]" is incompatible with base class "CoContra" (reportGeneralTypeIssues)
-generics_variance.py:164:33 - error: Type "Co[Contra[T_contra@CoToContraToContra]]" cannot be assigned to type variable "T_contra@Contra"
+generics_variance.py:163:33 - error: Type "Co[Contra[T_contra@CoToContraToContra]]" cannot be assigned to type variable "T_contra@Contra"
   Variance of type argument "Co[Contra[T_contra@CoToContraToContra]]" is incompatible with base class "Contra" (reportGeneralTypeIssues)
-generics_variance.py:168:37 - error: Type "Contra[Contra[T_co@ContraToContraToContra]]" cannot be assigned to type variable "T_contra@Contra"
+generics_variance.py:167:37 - error: Type "Contra[Contra[T_co@ContraToContraToContra]]" cannot be assigned to type variable "T_contra@Contra"
   Variance of type argument "Contra[Contra[T_co@ContraToContraToContra]]" is incompatible with base class "Contra" (reportGeneralTypeIssues)
-generics_variance.py:192:43 - error: Could not specialize type "Contra_TA[T_contra@Contra_TA]"
+generics_variance.py:191:43 - error: Could not specialize type "Contra_TA[T_contra@Contra_TA]"
   Variance of type argument "Co_TA[Contra_TA[T_contra@CoToContraToContra_WithTA]]" is incompatible with "T_contra@Contra_TA"
-generics_variance.py:197:15 - error: Could not specialize type "Contra_TA[T_contra@Contra_TA]"
+generics_variance.py:196:15 - error: Could not specialize type "Contra_TA[T_contra@Contra_TA]"
   Variance of type argument "Contra_TA[Contra_TA[T_co@ContraToContraToContra_WithTA]]" is incompatible with "T_contra@Contra_TA"
 """

conformance/tests/generics_variance.py

@@ -52,10 +52,9 @@ def dump_employee(e: E) -> E:
 B_co = TypeVar("B_co", covariant=True)
 
 
-# > Variance is only applicable to generic types; generic functions do not
-# > have this property. The latter should be defined using only type variables
-# > without covariant or contravariant keyword arguments.
-def bad_func(x: list[B_co]) -> B_co:  # Type checker error
+# > Variance has no meaning, and should therefore be ignored by type checkers,
+# > if a type variable is bound to a generic function or type alias.
+def func(x: list[B_co]) -> B_co:  # OK
     ...
 
 

docs/spec/generics.rst

@@ -562,32 +562,21 @@ mutable collection classes (e.g. ``MutableMapping`` and
 a contravariant type is the ``Generator`` type, which is contravariant
 in the ``send()`` argument type (see below).
 
-Note: Covariance or contravariance is *not* a property of a type variable,
-but a property of a generic class defined using this variable.
-Variance is only applicable to generic types; generic functions
-do not have this property. The latter should be defined using only
-type variables without ``covariant`` or ``contravariant`` keyword arguments.
-For example, the following example is
-fine::
+Variance is meaningful only when a type variable is bound to a generic class.
+Variance has no meaning, and should therefore be ignored by type checkers, if
+a type variable is bound to a generic function or type alias.
 
-  from typing import TypeVar
-
-  class Employee: ...
-
-  class Manager(Employee): ...
-
-  E = TypeVar('E', bound=Employee)
+For example::
 
-  def dump_employee(e: E) -> None: ...
+  T = TypeVar('T', covariant=True)
 
-  dump_employee(Manager())  # OK
+  class A(Generic[T]):  # T is covariant in this context
+    ...
 
-while the following is prohibited::
+  def f(x: T) -> None:  # Variance of T is meaningless in this context
+    ...
 
-  B_co = TypeVar('B_co', covariant=True)
-
-  def bad_func(x: B_co) -> B_co:  # Flagged as error by a type checker
-      ...
+  Alias = list[T] | set[T]  # Variance of T is meaningless in this context
 
 .. _`paramspec`: