Narrow Any in and_conditional_map (#21167)

Prior to this, mypy's behaviour is inconsistent depending on the order
of the condition or the presence of terms in the condition other than
`isinstance`

Fixes #17100 , fixes #20363 , fixes #16462

Author: hauntsaninja

mypy/checker.py

@@ -8633,15 +8633,19 @@ def and_conditional_maps(m1: TypeMap, m2: TypeMap, *, use_meet: bool = False) ->
     the truth of e2.
     """
     # Both conditions can be true; combine the information. Anything
-    # we learn from either conditions' truth is valid. If the same
-    # expression's type is refined by both conditions, we somewhat
-    # arbitrarily give precedence to m2 unless m1 value is Any.
-    # In the future, we could use an intersection type or meet_types().
+    # we learn from either conditions' truth is valid.
+    # If the same expression's type is refined by both conditions and use_meet=False, we somewhat
+    # arbitrarily give precedence to m2 unless m2's value is Any or m1's value is Never.
     result = m2.copy()
-    m2_keys = {literal_hash(n2) for n2 in m2}
+    m2_exprs = {literal_hash(n2): n2 for n2 in m2}
     for n1 in m1:
-        if literal_hash(n1) not in m2_keys or isinstance(
-            get_proper_type(m1[n1]), (AnyType, UninhabitedType)
+        n1_hash = literal_hash(n1)
+        if n1_hash not in m2_exprs or (
+            not use_meet
+            and (
+                isinstance(get_proper_type(m1[n1]), UninhabitedType)
+                or isinstance(get_proper_type(m2[m2_exprs[n1_hash]]), AnyType)
+            )
         ):
             result[n1] = m1[n1]
     if use_meet:

test-data/unit/check-isinstance.test

@@ -1478,6 +1478,26 @@ def f(x: Any):
     x + "foo"  # E: Unsupported operand types for + ("int" and "str")
 [builtins fixtures/isinstance.pyi]
 
+[case testIsinstanceNarrowingAny]
+# flags: --warn-unreachable
+from typing import Any
+
+class A: pass
+class B(A): pass
+
+def f1(x: Any) -> None:
+    if x and isinstance(x, A):
+        reveal_type(x)  # N: Revealed type is "__main__.A"
+    if isinstance(x, A) and x:
+        reveal_type(x)  # N: Revealed type is "__main__.A"
+
+def f2(x: Any) -> None:
+    if isinstance(x, A) and not isinstance(x, B):
+        reveal_type(x)  # N: Revealed type is "__main__.A"
+    if not isinstance(x, B) and isinstance(x, A):
+        reveal_type(x)  # N: Revealed type is "__main__.A"
+[builtins fixtures/isinstance.pyi]
+
 [case testIsinstanceOfGenericClassRetainsParameters]
 # flags: --warn-unreachable
 from typing import List, Union

test-data/unit/check-narrowing.test

@@ -1727,7 +1727,7 @@ reveal_type(c)  # N: Revealed type is "__main__.C"
 
 c1: C
 if isinstance(c1, tp) and isinstance(c1, D):
-    reveal_type(c1)  # N: Revealed type is "Any"
+    reveal_type(c1)  # N: Revealed type is "__main__.D"
 else:
     reveal_type(c1)  # N: Revealed type is "__main__.C"
 reveal_type(c1)  # N: Revealed type is "__main__.C"
@@ -1741,7 +1741,7 @@ reveal_type(c2)  # N: Revealed type is "__main__.C"
 
 c3: C
 if isinstance(c3, D) and isinstance(c3, tp):
-    reveal_type(c3)  # N: Revealed type is "Any"
+    reveal_type(c3)  # N: Revealed type is "__main__.D"
 else:
     reveal_type(c3)  # N: Revealed type is "__main__.C"
 reveal_type(c3)  # N: Revealed type is "__main__.C"