Added special-case handling for instance variables in a dataclass tha…

…t are marked `Final`. Previously, these were flagged as an error because there was no explicit value assigned to them, but the synthesized `__init__` method implicitly initializes them.

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

@@ -2122,7 +2122,28 @@ export class Checker extends ParseTreeWalker {
         if (!sawAssignment && !this._fileInfo.isStubFile) {
             const firstDecl = decls.find((decl) => decl.type === DeclarationType.Variable && decl.isFinal);
             if (firstDecl) {
-                this._evaluator.addError(Localizer.Diagnostic.finalUnassigned().format({ name }), firstDecl.node);
+                // Is this an instance variable declared within a dataclass? If so, it
+                // is implicitly initialized by the synthesized `__init__` method and
+                // therefore has an implied assignment.
+                let isImplicitlyAssigned = false;
+
+                if (symbol.isClassMember() && !symbol.isClassVar()) {
+                    const containingClass = ParseTreeUtils.getEnclosingClass(firstDecl.node, /* stopAtFunction */ true);
+                    if (containingClass) {
+                        const classType = this._evaluator.getTypeOfClass(containingClass);
+                        if (
+                            classType &&
+                            isClass(classType.decoratedType) &&
+                            ClassType.isDataClass(classType.decoratedType)
+                        ) {
+                            isImplicitlyAssigned = true;
+                        }
+                    }
+                }
+
+                if (!isImplicitlyAssigned) {
+                    this._evaluator.addError(Localizer.Diagnostic.finalUnassigned().format({ name }), firstDecl.node);
+                }
             }
         }
     }

packages/pyright-internal/src/tests/samples/final5.py

@@ -0,0 +1,18 @@
+# This sample tests that instance variables declared as Final within
+# a dataclass do not need to have an explicit assignment because
+# the generated __init__ method will assign them.
+
+from dataclasses import dataclass
+from typing import Final
+
+
+class Foo1:
+    x: Final[int]
+
+    def __init__(self, x: int) -> None:
+        self.x = x
+
+
+@dataclass
+class Foo2:
+    x: Final[int]

packages/pyright-internal/src/tests/typeEvaluator4.test.ts

@@ -310,6 +310,11 @@ test('Final4', () => {
     TestUtils.validateResults(analysisResults, 3);
 });
 
+test('Final5', () => {
+    const analysisResults = TestUtils.typeAnalyzeSampleFiles(['final5.py']);
+    TestUtils.validateResults(analysisResults, 0);
+});
+
 test('InferredTypes1', () => {
     const analysisResults = TestUtils.typeAnalyzeSampleFiles(['inferredTypes1.py']);
     TestUtils.validateResults(analysisResults, 0);