Fixed bug that resulted in a false positive error relating to variance incompatibility. This addresses https://github.com/microsoft/pyright/issues/4111.

packages/pyright-internal/src/analyzer/typeEvaluator.ts

@@ -7175,7 +7175,8 @@ export function createTypeEvaluator(importLookup: ImportLookup, evaluatorOptions
                 EvaluatorFlags.DoNotSpecialize |
                 EvaluatorFlags.DisallowParamSpec |
                 EvaluatorFlags.DisallowTypeVarTuple |
-                EvaluatorFlags.AllowRequired
+                EvaluatorFlags.AllowRequired |
+                EvaluatorFlags.EnforceTypeVarVarianceConsistency
             );
 
             if (!options?.isAnnotatedClass) {

packages/pyright-internal/src/localization/package.nls.en-us.json

@@ -676,8 +676,8 @@
         "useListInstead": "Use List[T] to indicate a list type or Union[T1, T2] to indicate a union type",
         "useTupleInstead": "Use Tuple[T1, ..., Tn] to indicate a tuple type or Union[T1, T2] to indicate a union type",
         "useTypeInstead": "Use Type[T] instead",
-        "varianceMismatchForClass": "Variance of TypeVar \"{typeVarName}\" is incompatible with base class \"{className}\"",
-        "varianceMismatchForTypeAlias": "Variance of TypeVar \"{typeVarName}\" is incompatible with \"{typeAliasParam}\""
+        "varianceMismatchForClass": "Variance of type argument \"{typeVarName}\" is incompatible with base class \"{className}\"",
+        "varianceMismatchForTypeAlias": "Variance of type argument \"{typeVarName}\" is incompatible with \"{typeAliasParam}\""
     },
     "CodeAction": {
         "addOptionalToAnnotation": "Add \"Optional\" to Type Annotation",

packages/pyright-internal/src/tests/samples/genericTypes97.py

@@ -60,8 +60,6 @@ class Class5(Generic[T_contra]):
     ...
 
 
-# This should generate an error because frozenset
-# takes a covariant type parameter.
 class Class5_Child1(Class5[frozenset[T_contra]]):
     ...
 
@@ -105,3 +103,31 @@ class Class6_Child4(Class6[T, T]):
 # compatible with T_contra.
 class Class6_Child5(Class6[Sequence[T_co], Sequence[T_co]]):
     ...
+
+
+class Co(Generic[T_co]):
+    ...
+
+
+class Contra(Generic[T_contra]):
+    ...
+
+
+class CoToContra(Generic[T_contra]):
+    def f(self, arg: Co[T_contra]) -> None:
+        ...
+
+
+class ContraToContra(Generic[T_co]):
+    def f(self, arg: Contra[T_co]) -> None:
+        ...
+
+
+class CoToCo(Generic[T_co]):
+    def f(self) -> Co[T_co]:
+        ...
+
+
+class ContraToCo(Generic[T_contra]):
+    def f(self) -> Contra[T_contra]:
+        ...

packages/pyright-internal/src/tests/samples/typeAlias18.py

@@ -33,13 +33,13 @@ class A_3(A[T2]):
     ...
 
 
-A_Alias_3 = A[T2] | Callable[[T3], T2]
+class B(Generic[T1, T2]):
+    pass
 
 
-class B(list[A_Alias_3[T1, T2]]):
-    ...
+B_Alias_1 = B[T2, T3]
 
 
 # This should generate an error because the variance is incompatible.
-class C(list[A_Alias_3[T3, T3]]):
+class C(B_Alias_1[T3, T2]):
     ...

packages/pyright-internal/src/tests/typeEvaluator2.test.ts

@@ -981,7 +981,7 @@ test('GenericTypes96', () => {
 test('GenericTypes97', () => {
     const analysisResults = TestUtils.typeAnalyzeSampleFiles(['genericTypes97.py']);
 
-    TestUtils.validateResults(analysisResults, 9);
+    TestUtils.validateResults(analysisResults, 8);
 });
 
 test('GenericTypes98', () => {