Avoid treating pass and ... as no-op for reachability (#20488)

Fixes #16232

The inclusion of these statements dates back to the original
`--warn-unreachable` PR (and doesn't provide justification)

I've been advocating for more consistent heuristics for allowable
unreachable code across type checkers and I can't come up with a good
reason that this should be allowed

Author: hauntsaninja

mypy/checker.py

@@ -3181,12 +3181,10 @@ def is_noop_for_reachability(self, s: Statement) -> bool:
             return True
         elif isinstance(s, ReturnStmt) and is_literal_not_implemented(s.expr):
             return True
-        elif isinstance(s, (RaiseStmt, PassStmt)):
+        elif isinstance(s, RaiseStmt):
             return True
         elif isinstance(s, ExpressionStmt):
-            if isinstance(s.expr, EllipsisExpr):
-                return True
-            elif isinstance(s.expr, CallExpr):
+            if isinstance(s.expr, CallExpr):
                 with self.expr_checker.msg.filter_errors(filter_revealed_type=True):
                     typ = get_proper_type(
                         self.expr_checker.accept(

test-data/unit/check-unreachable-code.test

@@ -1637,7 +1637,17 @@ class C:
             return C()
         else:
             return NotImplemented
+[builtins fixtures/isinstance.pyi]
+
+[case testPassAndEllipsisUnreachable]
+# flags: --warn-unreachable
+def f() -> None:
+    return
+    pass  # E: Statement is unreachable
 
+def g() -> None:
+    return
+    ...  # E: Statement is unreachable
 [builtins fixtures/isinstance.pyi]
 
 [case testUnreachableStatementPrettyHighlighting]