Fixed a bug that led to incorrect type evaluation when a constrained TypeVar includes literal types as constraints. This addresses #6917. (#6929)

.vscode/settings.json

@@ -12,4 +12,5 @@
         "source.organizeImports": "explicit"
     },
     "typescript.tsdk": "node_modules/typescript/lib",
+    "python.languageServer": "None",
 }

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

@@ -52,6 +52,7 @@ import {
     getTypeCondition,
     getTypeVarScopeId,
     isEffectivelyInstantiable,
+    isLiteralTypeOrUnion,
     isPartlyUnknown,
     mapSubtypes,
     sortTypes,
@@ -652,6 +653,7 @@ function assignTypeToConstrainedTypeVar(
 
     const curWideTypeBound = curEntry?.wideBound;
     const curNarrowTypeBound = curEntry?.narrowBound;
+    let forceRetainLiterals = false;
 
     if (isTypeVar(srcType)) {
         if (
@@ -785,6 +787,8 @@ function assignTypeToConstrainedTypeVar(
             })
         );
         return false;
+    } else if (isLiteralTypeOrUnion(constrainedType)) {
+        forceRetainLiterals = true;
     }
 
     if (curNarrowTypeBound && !isAnyOrUnknown(curNarrowTypeBound)) {
@@ -829,7 +833,14 @@ function assignTypeToConstrainedTypeVar(
     } else {
         // Assign the type to the type var.
         if (!typeVarContext.isLocked() && isTypeVarInScope) {
-            updateTypeVarType(evaluator, typeVarContext, destType, constrainedType, curWideTypeBound);
+            updateTypeVarType(
+                evaluator,
+                typeVarContext,
+                destType,
+                constrainedType,
+                curWideTypeBound,
+                forceRetainLiterals
+            );
         }
     }
 

packages/pyright-internal/src/tests/samples/constrainedTypeVar19.py

@@ -0,0 +1,18 @@
+# This tests the handling of a constrained TypeVar with literal types
+# in the constraints.
+
+from typing import TypeVar, Literal, Generic
+
+T = TypeVar("T", Literal[True], Literal[False])
+
+
+class A(Generic[T]):
+    def __init__(self, null: T = False) -> None:
+        pass
+
+
+A(null=bool())  # Type error
+
+reveal_type(A(null=False), expected_text="A[Literal[False]]")
+reveal_type(A(), expected_text="A[Literal[False]]")
+reveal_type(A(null=True), expected_text="A[Literal[True]]")

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

@@ -518,6 +518,12 @@ test('ConstrainedTypeVar18', () => {
     TestUtils.validateResults(analysisResults, 0);
 });
 
+test('ConstrainedTypeVar19', () => {
+    const analysisResults = TestUtils.typeAnalyzeSampleFiles(['constrainedTypeVar19.py']);
+
+    TestUtils.validateResults(analysisResults, 1);
+});
+
 test('MissingTypeArg1', () => {
     const configOptions = new ConfigOptions(Uri.empty());