Modified is None type guard logic to eliminate Any or Unknown i…

…n the positive narrowing case. This addresses #5508.

docs/type-concepts-advanced.md

@@ -200,7 +200,7 @@ This “narrowing for implied else” technique works for all narrowing expressi
 
 ### Narrowing Any
 
-In general, the type `Any` is not narrowed. The only exceptions to this rule are the built-in `isinstance` and `issubclass` type guards, class pattern matching in “match” statements, and user-defined type guards. In all other cases, `Any` is left as is, even for assignments.
+In general, the type `Any` is not narrowed. The only exceptions to this rule are the built-in `isinstance`, `issubclass`, and `is None` type guards, class pattern matching in “match” statements, and user-defined type guards. In all other cases, `Any` is left as is, even for assignments.
 
 ```python
 a: Any = 3

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

@@ -1002,9 +1002,9 @@ function narrowTypeForIsNone(evaluator: TypeEvaluator, type: Type, isPositiveTes
         /* conditionFilter */ undefined,
         (subtype, unexpandedSubtype) => {
             if (isAnyOrUnknown(subtype)) {
-                // We need to assume that "Any" is always both None and not None,
-                // so it matches regardless of whether the test is positive or negative.
-                return subtype;
+                // If it's an Any or Unknown, we can definitively say that it's
+                // a None based on this type guard.
+                return isPositiveTest ? NoneType.createInstance() : subtype;
             }
 
             // If this is a TypeVar that isn't constrained, use the unexpanded

packages/pyright-internal/src/tests/samples/typeNarrowingIsNone1.py

@@ -4,7 +4,7 @@
 
 # pyright: strict, reportUnusedVariable=false
 
-from typing import TypeVar
+from typing import Any, Literal, Protocol, TypeVar
 
 
 def func1(x: int | None):
@@ -54,3 +54,29 @@ def func4(x: _T2) -> _T2:
     else:
         reveal_type(x, expected_text="_T2@func4")
         return x
+
+
+def func5(x: Any | None):
+    if x is None:
+        reveal_type(x, expected_text="None")
+    else:
+        reveal_type(x, expected_text="Any")
+
+
+def func6(x: Any | object | None):
+    if x is None:
+        reveal_type(x, expected_text="None")
+    else:
+        reveal_type(x, expected_text="Any | object")
+
+
+class NoneProto(Protocol):
+    def __bool__(self) -> Literal[False]:
+        ...
+
+
+def func7(x: NoneProto | None):
+    if x is None:
+        reveal_type(x, expected_text="None")
+    else:
+        reveal_type(x, expected_text="NoneProto")