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

Fixes https://github.com/python/mypy/issues/18967
Fixes https://github.com/python/mypy/issues/16494
Fixes https://github.com/python/mypy/issues/15793
Fixes https://github.com/python/mypy/issues/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).

mypy/checkexpr.py

diff --git a/mypy/checkexpr.py b/mypy/checkexpr.py
index 3fa4df2..008e056 100644
--- a/mypy/checkexpr.py
+++ b/mypy/checkexpr.py
@@ -6277,7 +6277,13 @@
                     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

diff --git a/test-data/unit/check-isinstance.test b/test-data/unit/check-isinstance.test
index 49140bf..058db1e 100644
--- a/test-data/unit/check-isinstance.test
+++ b/test-data/unit/check-isinstance.test
@@ -1812,9 +1812,9 @@
 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

diff --git a/test-data/unit/check-narrowing.test b/test-data/unit/check-narrowing.test
index 1856ca2..dc2cfd4 100644
--- a/test-data/unit/check-narrowing.test
+++ b/test-data/unit/check-narrowing.test
@@ -1284,7 +1284,7 @@
         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 @@
     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

diff --git a/test-data/unit/check-python310.test b/test-data/unit/check-python310.test
index 016f505..a25a7b7 100644
--- a/test-data/unit/check-python310.test
+++ b/test-data/unit/check-python310.test
@@ -1299,7 +1299,7 @@
 
 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