[PEP 695] Inherit variance if base class has explicit variance (#17787)

Previously we only inferred variance based on member types, but if a
base class has explicit variance for some type variables, we need to
consider it as well.

Author: JukkaL

mypy/subtypes.py

@@ -2031,6 +2031,15 @@ def infer_variance(info: TypeInfo, i: int) -> bool:
                     contra = False
                     if settable:
                         co = False
+
+        # Infer variance from base classes, in case they have explicit variances
+        for base in info.bases:
+            base2 = expand_type(base, {tvar.id: object_type})
+            if not is_subtype(base, base2):
+                co = False
+            if not is_subtype(base2, base):
+                contra = False
+
         if co:
             v = COVARIANT
         elif contra:

test-data/unit/check-python312.test

@@ -1891,3 +1891,43 @@ class A:
 
 a = A()
 v = a.f
+
+[case testPEP695VarianceInheritedFromBaseWithExplicitVariance]
+# flags: --enable-incomplete-feature=NewGenericSyntax
+from typing import TypeVar, Generic
+
+T = TypeVar("T")
+
+class ParentInvariant(Generic[T]):
+    pass
+
+class Invariant1[T](ParentInvariant[T]):
+    pass
+
+a1: Invariant1[int] = Invariant1[float]()  # E: Incompatible types in assignment (expression has type "Invariant1[float]", variable has type "Invariant1[int]")
+a2: Invariant1[float] = Invariant1[int]()  # E: Incompatible types in assignment (expression has type "Invariant1[int]", variable has type "Invariant1[float]")
+
+T_contra = TypeVar("T_contra", contravariant=True)
+
+class ParentContravariant(Generic[T_contra]):
+    pass
+
+class Contravariant[T](ParentContravariant[T]):
+    pass
+
+b1: Contravariant[int] = Contravariant[float]()
+b2: Contravariant[float] = Contravariant[int]()  # E: Incompatible types in assignment (expression has type "Contravariant[int]", variable has type "Contravariant[float]")
+
+class Invariant2[T](ParentContravariant[T]):
+    def f(self) -> T: ...
+
+c1: Invariant2[int] = Invariant2[float]()  # E: Incompatible types in assignment (expression has type "Invariant2[float]", variable has type "Invariant2[int]")
+c2: Invariant2[float] = Invariant2[int]()  # E: Incompatible types in assignment (expression has type "Invariant2[int]", variable has type "Invariant2[float]")
+
+class Multi[T, S](ParentInvariant[T], ParentContravariant[S]):
+    pass
+
+d1: Multi[int, str] = Multi[float, str]()  # E: Incompatible types in assignment (expression has type "Multi[float, str]", variable has type "Multi[int, str]")
+d2: Multi[float, str] = Multi[int, str]()  # E: Incompatible types in assignment (expression has type "Multi[int, str]", variable has type "Multi[float, str]")
+d3: Multi[str, int] = Multi[str, float]()
+d4: Multi[str, float] = Multi[str, int]()  # E: Incompatible types in assignment (expression has type "Multi[str, int]", variable has type "Multi[str, float]")

test-data/unit/fixtures/tuple-simple.pyi

@@ -5,7 +5,7 @@
 
 from typing import Iterable, TypeVar, Generic
 
-T = TypeVar('T')
+T = TypeVar('T', covariant=True)
 
 class object:
     def __init__(self): pass