Do not narrow types to Never with binder (#18972)

Fixes #18967
Fixes #16494
Fixes #15793
Fixes #12949

As you can see from updated test cases, it is kind of gray area, so
whether we go this way will depend on the `mypy_primer` results (and
also potentially on Dropbox internal code bases, where the above issue
may cause problems).

Author: ilevkivskyi

mypy/checkexpr.py

@@ -6297,7 +6297,13 @@ def narrow_type_from_binder(
                     known_type, restriction, prohibit_none_typevar_overlap=True
                 ):
                     return None
-                return narrow_declared_type(known_type, restriction)
+                narrowed = narrow_declared_type(known_type, restriction)
+                if isinstance(get_proper_type(narrowed), UninhabitedType):
+                    # If we hit this case, it means that we can't reliably mark the code as
+                    # unreachable, but the resulting type can't be expressed in type system.
+                    # Falling back to restriction is more intuitive in most cases.
+                    return restriction
+                return narrowed
         return known_type
 
     def has_abstract_type_part(self, caller_type: ProperType, callee_type: ProperType) -> bool:

test-data/unit/check-isinstance.test

@@ -1812,9 +1812,9 @@ reveal_type(fm)  # N: Revealed type is "__main__.FooMetaclass"
 if issubclass(fm, Foo):
     reveal_type(fm)  # N: Revealed type is "Type[__main__.Foo]"
 if issubclass(fm, Bar):
-    reveal_type(fm)  # N: Revealed type is "Never"
+    reveal_type(fm)  # N: Revealed type is "Type[__main__.Bar]"
 if issubclass(fm, Baz):
-    reveal_type(fm)  # N: Revealed type is "Never"
+    reveal_type(fm)  # N: Revealed type is "Type[__main__.Baz]"
 [builtins fixtures/isinstance.pyi]
 
 [case testIsinstanceAndNarrowTypeVariable]

test-data/unit/check-narrowing.test

@@ -1284,7 +1284,7 @@ def f(t: Type[T], a: A, b: B) -> None:
         reveal_type(a)  # N: Revealed type is "__main__.A"
 
     if type(b) is t:
-        reveal_type(b)  # N: Revealed type is "Never"
+        reveal_type(b)  # N: Revealed type is "T`-1"
     else:
         reveal_type(b)  # N: Revealed type is "__main__.B"
 
@@ -2413,3 +2413,14 @@ def foo(x: T) -> T:
     reveal_type(x)  # N: Revealed type is "T`-1"
     return x
 [builtins fixtures/isinstance.pyi]
+
+[case testDoNotNarrowToNever]
+def any():
+    return 1
+
+def f() -> None:
+    x = "a"
+    x = any()
+    assert isinstance(x, int)
+    reveal_type(x)  # N: Revealed type is "builtins.int"
+[builtins fixtures/isinstance.pyi]

test-data/unit/check-python310.test

@@ -1347,7 +1347,7 @@ m: str
 
 match m:
     case a if a := 1:  # E: Incompatible types in assignment (expression has type "int", variable has type "str")
-        reveal_type(a)  # N: Revealed type is "Never"
+        reveal_type(a)  # N: Revealed type is "Literal[1]?"
 
 [case testMatchAssigningPatternGuard]
 m: str