Retain None (unreachable) when typemap is None with type(x) is Foo check (python#18486)

Fixes python#18428. 

Prevents rewriting `None` ("unreachable") typemaps as empty dicts
("nothing to infer")

Author: sterliakov

mypy/checker.py

@@ -6021,6 +6021,8 @@ def is_type_call(expr: CallExpr) -> bool:
 
         def combine_maps(list_maps: list[TypeMap]) -> TypeMap:
             """Combine all typemaps in list_maps into one typemap"""
+            if all(m is None for m in list_maps):
+                return None
             result_map = {}
             for d in list_maps:
                 if d is not None:

test-data/unit/check-isinstance.test

@@ -2660,6 +2660,33 @@ y: Any
 if type(y) is int:
     reveal_type(y) # N: Revealed type is "builtins.int"
 
+[case testTypeEqualsCheckUsingIsNonOverlapping]
+# flags: --warn-unreachable
+from typing import Union
+
+y: str
+if type(y) is int:  # E: Subclass of "str" and "int" cannot exist: would have incompatible method signatures
+    y # E: Statement is unreachable
+else:
+    reveal_type(y) # N: Revealed type is "builtins.str"
+[builtins fixtures/isinstance.pyi]
+
+[case testTypeEqualsCheckUsingIsNonOverlappingChild-xfail]
+# flags: --warn-unreachable
+from typing import Union
+
+class A: ...
+class B: ...
+class C(A): ...
+x: Union[B, C]
+# C instance cannot be exactly its parent A, we need reversed subtyping relationship
+# here (type(parent) is Child).
+if type(x) is A:
+    reveal_type(x)  # E: Statement is unreachable
+else:
+    reveal_type(x)  # N: Revealed type is "Union[__main__.B, __main__.C]"
+[builtins fixtures/isinstance.pyi]
+
 [case testTypeEqualsNarrowingUnionWithElse]
 from typing import Union
 

test-data/unit/check-typevar-values.test

@@ -732,3 +732,16 @@ def foo3(x: NT) -> None:
 def foo4(x: NT) -> None:
     p, q = 1, 2.0  # type: (int, float)
 [builtins fixtures/tuple.pyi]
+
+[case testTypeVarValuesNarrowing]
+from typing import TypeVar
+
+W = TypeVar("W", int, str)
+
+def fn(w: W) -> W:
+    if type(w) is str:
+        reveal_type(w)  # N: Revealed type is "builtins.str"
+    elif type(w) is int:
+        reveal_type(w)  # N: Revealed type is "builtins.int"
+    return w
+[builtins fixtures/isinstance.pyi]