Add an option to enable checking unreachable code

Author: A5rocks

docs/source/config_file.rst

@@ -579,6 +579,13 @@ section of the command line docs.
 
     For example, it would report an error for :code:`def f(a: int, b)` but not :code:`def f(a, b)`.
 
+.. confval:: check_unreachable
+
+    :type: boolean
+    :default: False
+
+    Type-checks unreachable code.
+
 .. confval:: check_untyped_defs
 
     :type: boolean

mypy/checker.py

@@ -470,10 +470,16 @@ def check_first_pass(self) -> None:
                         if not reported_unreachable and self.binder.is_unreachable():
                             if not self.should_report_unreachable_issues():
                                 reported_unreachable = True
+                                if not self.options.check_unreachable:
+                                    break
+
                             elif not self.is_noop_for_reachability(d):
                                 self.msg.unreachable_statement(d)
                                 self.binder.emitted_unreachable_warning()
                                 reported_unreachable = True
+                                if not self.options.check_unreachable:
+                                    break
+
                         self.accept(d)
 
                 assert not self.current_node_deferred
@@ -3129,10 +3135,16 @@ def visit_block(self, b: Block) -> None:
                     break
                 elif not self.should_report_unreachable_issues():
                     reported_unreachable = True
+                    if not self.options.check_unreachable:
+                        break
+
                 elif not self.is_noop_for_reachability(s):
                     self.msg.unreachable_statement(s)
                     self.binder.emitted_unreachable_warning()
                     reported_unreachable = True
+                    if not self.options.check_unreachable:
+                        break
+
             self.accept(s)
 
     def should_report_unreachable_issues(self) -> bool:

mypy/checkexpr.py

@@ -5925,11 +5925,11 @@ def analyze_cond_branch(
         allow_none_return: bool = False,
         suppress_unreachable_errors: bool | None = None,
     ) -> Type:
-        # TODO: default based on flag (default to `True` if flag is not passed)
         unreachable_errors_suppressed = (
             suppress_unreachable_errors
             if suppress_unreachable_errors is not None
-            else self.chk.binder.is_unreachable_warning_suppressed()
+            else not self.chk.options.check_unreachable
+            or self.chk.binder.is_unreachable_warning_suppressed()
         )
         with self.chk.binder.frame_context(can_skip=True, fall_through=0):
             self.chk.push_type_map(map)

mypy/main.py

@@ -910,6 +910,14 @@ def add_invertible_flag(
         group=strictness_group,
     )
 
+    add_invertible_flag(
+        "--check-unreachable",
+        default=False,
+        strict_flag=False,
+        help="Type check unreachable code",
+        group=strictness_group,
+    )
+
     strict_help = "Strict mode; enables the following flags: {}".format(
         ", ".join(strict_flag_names)
     )

mypy/options.py

@@ -26,6 +26,7 @@ class BuildType:
     "allow_untyped_globals",
     "always_false",
     "always_true",
+    "check_unreachable",
     "check_untyped_defs",
     "debug_cache",
     "disable_error_code",
@@ -43,8 +44,8 @@ class BuildType:
     "enable_error_code",
     "enabled_error_codes",
     "extra_checks",
-    "follow_imports_for_stubs",
     "follow_imports",
+    "follow_imports_for_stubs",
     "follow_untyped_imports",
     "ignore_errors",
     "ignore_missing_imports",
@@ -159,6 +160,9 @@ 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
 

mypy/test/testcheck.py

@@ -140,6 +140,7 @@ def run_case_once(
             options.force_uppercase_builtins = True
         if "union-error" not in testcase.file:
             options.force_union_syntax = True
+        options.check_unreachable = not testcase.name.endswith("_skip_unreachable")
 
         if incremental_step and options.incremental:
             # Don't overwrite # flags: --no-incremental in incremental test cases

test-data/unit/check-errorcodes.test

@@ -1245,3 +1245,7 @@ def process(response1: int,response2: int) -> int: # E: Overloaded function sign
 
 def process(response1,response2)-> Union[float,int]:
    return response1 + response2
+
+[case testUnreachableCodeSkipped_skip_unreachable]
+if False:
+    reveal_type("x")  # will not output anything