Add a secret --check-unreachable (#21834)

Replaces #18707, references
#21835.

This is very unbaked, but I figure it's best to get the smallest bit in.
I'll see if I can figure out stacked PRs to add extra stuff to this
(namely decreasing the number of false errors in unreachable code!), but
this is probably super easy to review and a good start!

Author: A5rocks

mypy/checker.py

@@ -604,18 +604,31 @@ def check_first_pass(self, recurse_into_functions: bool = True) -> None:
             with self.tscope.module_scope(self.tree.fullname):
                 with self.enter_partial_types(), self.binder.top_frame_context():
                     marked_unreachable = False
+                    reported_unreachable = False
                     for d in self.tree.defs:
                         if self.binder.is_unreachable():
+                            finish = False
                             if not marked_unreachable:
                                 self.mark_unreachable(self.tree.defs, after=d)
                                 marked_unreachable = True
                             if not self.should_report_unreachable_issues():
-                                break
-                            if not self.is_noop_for_reachability(d):
+                                finish = True
+                            if (
+                                not finish
+                                and not reported_unreachable
+                                and not self.is_noop_for_reachability(d)
+                            ):
                                 self.msg.unreachable_statement(d)
+                                finish = True
+                                reported_unreachable = True
+
+                            if finish and not self.options.check_unreachable:
                                 break
-                        else:
-                            self.accept(d)
+
+                            if not self.options.check_unreachable:
+                                continue
+
+                        self.accept(d)
 
                 assert not self.current_node_deferred
 
@@ -3303,20 +3316,33 @@ def visit_block(self, b: Block) -> None:
             self.binder.unreachable()
             return
         marked_unreachable = False
+        reported_unreachable = False
         for s in b.body:
             if self.binder.is_unreachable():
+                finish = False
                 if self.scope.top_level_function() is None and not marked_unreachable:
                     self.mark_unreachable(b.body, after=s)
                     marked_unreachable = True
                 if not self.should_report_unreachable_issues():
-                    break
-                if not self.is_noop_for_reachability(s):
+                    finish = True
+                if (
+                    not finish
+                    and not reported_unreachable
+                    and not self.is_noop_for_reachability(s)
+                ):
                     self.msg.unreachable_statement(s)
+                    finish = True
+                    reported_unreachable = True
+
+                if finish and not self.options.check_unreachable:
                     break
-            else:
-                self.accept(s)
-                # Clear expression cache after each statement to avoid unlimited growth.
-                self.expr_checker.expr_cache.clear()
+
+                if not self.options.check_unreachable:
+                    continue
+
+            self.accept(s)
+            # Clear expression cache after each statement to avoid unlimited growth.
+            self.expr_checker.expr_cache.clear()
 
     def should_report_unreachable_issues(self) -> bool:
         return (

mypy/main.py

@@ -965,6 +965,13 @@ def add_invertible_flag(
         group=strictness_group,
     )
 
+    add_invertible_flag(
+        "--check-unreachable",
+        default=False,
+        help=argparse.SUPPRESS,  # "Type check unreachable code",
+        group=strictness_group,
+    )
+
     strict_help = "Strict mode; enables the following flags: {}".format(
         ", ".join(strict_flag_names)
     )

mypy/options.py

@@ -29,6 +29,7 @@ class BuildType:
     "allow_untyped_globals",
     "always_false",
     "always_true",
+    "check_unreachable",
     "check_untyped_defs",
     "debug_cache",
     "disable_error_code",
@@ -173,6 +174,9 @@ def __init__(self) -> None:
         # Disallow defining incompletely typed functions
         self.disallow_incomplete_defs = False
 
+        # Type check unreachable code
+        self.check_unreachable = False
+
         # Type check unannotated functions
         self.check_untyped_defs = False
 

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

@@ -1744,3 +1744,10 @@ assert sys.platform == "win32"
 
 42 + "no way"  # type: ignore[operator]
 [builtins fixtures/isinstancelist.pyi]
+
+
+[case testCheckUnreachableBasics]
+# flags: --check-unreachable --warn-unreachable
+if False:
+    reveal_type(5)  # E: Statement is unreachable \
+                    # N: Revealed type is "Literal[5]?"