Fixed regression that results in a false positive when a traditional type alias (not using PEP 695 syntax) defines a union and is then used as a second argument to an isinstance or issubclass call. This addresses #8302. (#8303)

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

@@ -4617,6 +4617,14 @@ export function createTypeEvaluator(
             return type;
         }
 
+        // Isinstance treats traditional (non-PEP 695) type aliases that are unions
+        // as tuples of classes rather than unions.
+        if ((flags & EvalFlags.IsinstanceParam) !== 0) {
+            if (isUnion(type) && type.typeAliasInfo && !type.typeAliasInfo.isPep695Syntax) {
+                return type;
+            }
+        }
+
         return type.specialForm ?? type;
     }
 
@@ -12055,7 +12063,8 @@ export function createTypeEvaluator(
                       EvalFlags.NoParamSpec |
                       EvalFlags.NoTypeVarTuple |
                       EvalFlags.NoFinal |
-                      EvalFlags.NoSpecialize
+                      EvalFlags.NoSpecialize |
+                      EvalFlags.IsinstanceParam
                     : EvalFlags.NoFinal | EvalFlags.NoSpecialize;
                 const exprTypeResult = getTypeOfExpression(
                     argParam.argument.valueExpression,

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

@@ -158,6 +158,10 @@ export const enum EvalFlags {
     // other container classes).
     StripTupleLiterals = 1 << 28,
 
+    // Interpret the expression using the specialized behaviors associated
+    // with the second argument to isinstance and issubclass calls.
+    IsinstanceParam = 1 << 29,
+
     // Defaults used for evaluating the LHS of a call expression.
     CallBaseDefaults = NoSpecialize,
 

packages/pyright-internal/src/tests/samples/typeNarrowingIsinstance1.py

@@ -1,7 +1,7 @@
 # This sample exercises the type analyzer's isinstance type narrowing logic.
 
 from types import NoneType
-from typing import Generic, Protocol, Sized, TypeVar, Union, Any, runtime_checkable
+from typing import Any, Generic, Protocol, Sized, TypeVar, Union, runtime_checkable
 
 S = TypeVar("S")
 T = TypeVar("T")
@@ -151,11 +151,25 @@ def handler(node: Base1) -> Any:
             reveal_type(node.value, expected_text="Sub1_1")
 
 
-def func8(a: int | list[int] | dict[str, int] | None):
-    if isinstance(
-        a,
-        (str, (int, list, type(None))),
-    ):
+def func8a(a: int | list[int] | dict[str, int] | None):
+    if isinstance(a, (str, (int, list, type(None)))):
+        reveal_type(a, expected_text="int | list[int] | None")
+    else:
+        reveal_type(a, expected_text="dict[str, int]")
+
+
+def func8b(a: int | list[int] | dict[str, int] | None):
+    if isinstance(a, str | int | list | type(None)):
+        reveal_type(a, expected_text="int | list[int] | None")
+    else:
+        reveal_type(a, expected_text="dict[str, int]")
+
+
+TA1 = str | int | list | None
+
+
+def func8c(a: int | list[int] | dict[str, int] | None):
+    if isinstance(a, TA1):
         reveal_type(a, expected_text="int | list[int] | None")
     else:
         reveal_type(a, expected_text="dict[str, int]")
@@ -200,12 +214,12 @@ def func11(x: Proto1):
         reveal_type(x, expected_text="Proto1")
 
 
-TA1 = list["TA2"] | dict[str, "TA2"]
-TA2 = str | TA1
+TA2 = list["TA3"] | dict[str, "TA3"]
+TA3 = str | TA2
 
 
-def func12(x: TA2) -> None:
+def func12(x: TA3) -> None:
     if isinstance(x, dict):
-        reveal_type(x, expected_text="dict[str, str | list[TA2] | dict[str, TA2]]")
+        reveal_type(x, expected_text="dict[str, str | list[TA3] | dict[str, TA3]]")
     else:
-        reveal_type(x, expected_text="str | list[str | list[TA2] | dict[str, TA2]]")
+        reveal_type(x, expected_text="str | list[str | list[TA3] | dict[str, TA3]]")