Fixed a bug that caused incorrect type inference for parameters in un…

…annotated methods within child classes who derive from a generic parent class. This addresses a bug reported on stack overflow: https://stackoverflow.com/questions/76466874/trouble-specifying-type-parameter-when-inheriting-from-generic-class.

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

@@ -16884,6 +16884,15 @@ export function createTypeEvaluator(importLookup: ImportLookup, evaluatorOptions
                                 functionNode.parameters[paramIndex].category
                             );
 
+                            // If the parameter type is generic, specialize it in the context
+                            // of the child class.
+                            if (requiresSpecialization(inferredParamType) && isClass(baseClassMemberInfo.classType)) {
+                                const typeVarContext = buildTypeVarContextFromSpecializedClass(
+                                    baseClassMemberInfo.classType
+                                );
+                                inferredParamType = applySolvedTypeVars(inferredParamType, typeVarContext);
+                            }
+
                             const fileInfo = AnalyzerNodeInfo.getFileInfo(functionNode);
                             if (fileInfo.isInPyTypedPackage && !fileInfo.isStubFile) {
                                 inferredParamType = TypeBase.cloneForAmbiguousType(inferredParamType);

packages/pyright-internal/src/tests/samples/paramInference2.py

@@ -0,0 +1,20 @@
+# This sample tests the logic that infers parameter types based on
+# annotated base class methods when the base class is generic.
+
+
+from typing import Generic, TypeVar
+
+T = TypeVar("T")
+
+
+class Parent(Generic[T]):
+    def method1(self, a: T, b: list[T]) -> None:
+        ...
+
+
+class Child(Parent[float]):
+    def method1(self, a, b):
+        reveal_type(self, expected_text="Self@Child")
+        reveal_type(a, expected_text="float")
+        reveal_type(b, expected_text="list[float]")
+        return a

packages/pyright-internal/src/tests/typeEvaluator1.test.ts

@@ -1571,6 +1571,12 @@ test('ParamInference1', () => {
     TestUtils.validateResults(analysisResults, 0);
 });
 
+test('ParamInference2', () => {
+    const analysisResults = TestUtils.typeAnalyzeSampleFiles(['paramInference2.py']);
+
+    TestUtils.validateResults(analysisResults, 0);
+});
+
 test('Dictionary1', () => {
     const analysisResults = TestUtils.typeAnalyzeSampleFiles(['dictionary1.py']);