Enhanced isinstance narrowing logic to retain type arguments in cas…

…es where the filter type (the second argument) is a child of the un-narrowed type and the child has a type parameter that is used multiple times in its base class (e.g. `Child[T](Parent[T, T])`). This addresses #9141. (#9144)
microsoft · Oct 3, 2024 · cd5f5d3 · cd5f5d3
1 parent 3b8ffed
commit cd5f5d3
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Showing 2 changed files with 11 additions and 18 deletions.
diff --git a/packages/pyright-internal/src/analyzer/constraintSolver.ts b/packages/pyright-internal/src/analyzer/constraintSolver.ts
@@ -45,7 +45,6 @@ import {
     TypeVarKind,
     TypeVarScopeId,
     TypeVarType,
-    UnknownType,
     Variance,
 } from './types';
 import {
@@ -476,23 +475,17 @@ export function addConstraintsForExpectedType(
                         typeArgValue = transformExpectedType(typeArgValue, liveTypeVarScopes, usageOffset);
                     }
 
-                    if (typeArgValue) {
-                        const variance = TypeVarType.getVariance(typeVar);
-
-                        // If this type variable already has a type, don't overwrite it. This can
-                        // happen if a single type variable in the derived class is used multiple times
-                        // in the specialized base class type (e.g. Mapping[T, T]).
-                        if (constraints.getMainConstraintSet().getTypeVar(targetTypeVar)) {
-                            isResultValid = false;
-                            typeArgValue = UnknownType.create();
-                        }
-
-                        constraints.setBounds(
+                    if (
+                        !typeArgValue ||
+                        !assignTypeVar(
+                            evaluator,
                             targetTypeVar,
-                            variance === Variance.Covariant ? undefined : typeArgValue,
-                            variance === Variance.Contravariant ? undefined : typeArgValue
-                        );
-                    } else {
+                            typeArgValue,
+                            /* diag */ undefined,
+                            constraints,
+                            AssignTypeFlags.RetainLiteralsForTypeVar
+                        )
+                    ) {
                         isResultValid = false;
                     }
                 }

diff --git a/packages/pyright-internal/src/tests/samples/typeNarrowingIsinstance1.py b/packages/pyright-internal/src/tests/samples/typeNarrowingIsinstance1.py
@@ -208,7 +208,7 @@ class Sub2(Base2[T, T]):
 
 def func10(val: Sub2[str] | Base2[str, float]):
     if isinstance(val, Sub2):
-        reveal_type(val, expected_text="Sub2[str] | Sub2[Unknown]")
+        reveal_type(val, expected_text="Sub2[str] | Sub2[str | float]")
 
 
 @runtime_checkable