Fix partially defined in the case of missing type maps

Thanks @AlexWaygood for sending me on this adventure. This took me a
while to debug!

It turns out when we don't need to warn about unreachable code, we don't
end up calling `self.is_noop_for_reachability(s)` (which is meant to
tell us whether the code should be warned about or is
`raise AssertionError` or `typing.assert_never(never)` or something.
https://github.com/python/mypy/blob/6f650cff9ab21f81069e0ae30c92eae94219ea63/mypy/checker.py#L2748

This innocuous check has a side effect that turns out to be important
for the partially undefined checks. These checks work by reaching
into the type map populated by the checker. But if we never actually
ended up analysing the code, we never populate the type map.

This therefore changes things to assume that if we couldn't find the
expression in the type map, it's probably because it was unreachable.

mypy/partially_defined.py

@@ -506,7 +506,7 @@ def visit_break_stmt(self, o: BreakStmt) -> None:
         self.tracker.skip_branch()
 
     def visit_expression_stmt(self, o: ExpressionStmt) -> None:
-        if isinstance(self.type_map.get(o.expr, None), UninhabitedType):
+        if isinstance(self.type_map.get(o.expr, None), (UninhabitedType, type(None))):
             self.tracker.skip_branch()
         super().visit_expression_stmt(o)
 

test-data/unit/check-possibly-undefined.test

@@ -1026,3 +1026,20 @@ class B:
         else:
             # Same as above but in a loop.
             b = a  # E: Name "a" may be undefined
+
+[case testUnreachableCausingMissingTypeMap]
+# flags: --enable-error-code possibly-undefined --enable-error-code used-before-def --no-warn-unreachable
+# Regression test for https://github.com/python/mypy/issues/15958
+from typing import Union, NoReturn
+
+def assert_never(__x: NoReturn) -> NoReturn: ...
+
+def foo(x: Union[int, str]) -> None:
+    if isinstance(x, str):
+        f = "foo"
+    elif isinstance(x, int):
+        f = "bar"
+    else:
+        assert_never(x)
+    f  # OK
+[builtins fixtures/tuple.pyi]