Consistently avoid type-checking unreachable code (#15386)

- On module-level, now we'll skip remaining statements once unreachable.
This brings the behavior in line with function-level behavior.
- For module and function code, if `--warn-unreachable` is enabled,
we'll emit an error, just once, on the first unreachable statement
that's not a no-op statement. Previously a no-op statement would not
have the "Unreachable statement" error, but the subsequent statements
did not have the error either, e.g.
  ```diff
   raise Exception
   assert False  # no error since it's a "no-op statement"
  -foo = 42
  +foo = 42  # E: Unreachable statement
   spam = "ham"  # no error since we warn just once
  ```

mypy/checker.py

@@ -464,14 +464,14 @@ def check_first_pass(self) -> None:
             with self.tscope.module_scope(self.tree.fullname):
                 with self.enter_partial_types(), self.binder.top_frame_context():
                     for d in self.tree.defs:
-                        if (
-                            self.binder.is_unreachable()
-                            and self.should_report_unreachable_issues()
-                            and not self.is_raising_or_empty(d)
-                        ):
-                            self.msg.unreachable_statement(d)
-                            break
-                        self.accept(d)
+                        if self.binder.is_unreachable():
+                            if not self.should_report_unreachable_issues():
+                                break
+                            if not self.is_noop_for_reachability(d):
+                                self.msg.unreachable_statement(d)
+                                break
+                        else:
+                            self.accept(d)
 
                 assert not self.current_node_deferred
 
@@ -2706,10 +2706,13 @@ def visit_block(self, b: Block) -> None:
             return
         for s in b.body:
             if self.binder.is_unreachable():
-                if self.should_report_unreachable_issues() and not self.is_raising_or_empty(s):
+                if not self.should_report_unreachable_issues():
+                    break
+                if not self.is_noop_for_reachability(s):
                     self.msg.unreachable_statement(s)
-                break
-            self.accept(s)
+                    break
+            else:
+                self.accept(s)
 
     def should_report_unreachable_issues(self) -> bool:
         return (
@@ -2719,11 +2722,11 @@ def should_report_unreachable_issues(self) -> bool:
             and not self.binder.is_unreachable_warning_suppressed()
         )
 
-    def is_raising_or_empty(self, s: Statement) -> bool:
+    def is_noop_for_reachability(self, s: Statement) -> bool:
         """Returns 'true' if the given statement either throws an error of some kind
         or is a no-op.
 
-        We use this function mostly while handling the '--warn-unreachable' flag. When
+        We use this function while handling the '--warn-unreachable' flag. When
         that flag is present, we normally report an error on any unreachable statement.
         But if that statement is just something like a 'pass' or a just-in-case 'assert False',
         reporting an error would be annoying.

mypyc/test-data/run-misc.test

@@ -1108,25 +1108,14 @@ assert not C
 # make the initial import fail
 assert False
 
-class C:
-    def __init__(self):
-        self.x = 1
-        self.y = 2
-def test() -> None:
-    a = C()
 [file driver.py]
 # load native, cause PyInit to be run, create the module but don't finish initializing the globals
-try:
-    import native
-except:
-    pass
-try:
-    # try accessing those globals that were never properly initialized
-    import native
-    native.test()
-# should fail with AssertionError due to `assert False` in other function
-except AssertionError:
-    pass
+for _ in range(2):
+    try:
+        import native
+        raise RuntimeError('exception expected')
+    except AssertionError:
+        pass
 
 [case testRepeatedUnderscoreFunctions]
 def _(arg): pass

test-data/unit/check-classes.test

@@ -7725,10 +7725,14 @@ class D:
     def __new__(cls) -> NoReturn: ...
     def __init__(self) -> NoReturn: ...
 
-reveal_type(A())  # N: Revealed type is "<nothing>"
-reveal_type(B())  # N: Revealed type is "<nothing>"
-reveal_type(C())  # N: Revealed type is "<nothing>"
-reveal_type(D())  # N: Revealed type is "<nothing>"
+if object():
+    reveal_type(A())  # N: Revealed type is "<nothing>"
+if object():
+    reveal_type(B())  # N: Revealed type is "<nothing>"
+if object():
+    reveal_type(C())  # N: Revealed type is "<nothing>"
+if object():
+    reveal_type(D())  # N: Revealed type is "<nothing>"
 
 [case testOverloadedNewAndInitNoReturn]
 from typing import NoReturn, overload
@@ -7767,13 +7771,20 @@ class D:
     def __init__(self, a: int) -> None: ...
     def __init__(self, a: int = ...) -> None: ...
 
-reveal_type(A())  # N: Revealed type is "<nothing>"
+if object():
+    reveal_type(A())  # N: Revealed type is "<nothing>"
 reveal_type(A(1))  # N: Revealed type is "__main__.A"
-reveal_type(B())  # N: Revealed type is "<nothing>"
+
+if object():
+    reveal_type(B())  # N: Revealed type is "<nothing>"
 reveal_type(B(1))  # N: Revealed type is "__main__.B"
-reveal_type(C())  # N: Revealed type is "<nothing>"
+
+if object():
+    reveal_type(C())  # N: Revealed type is "<nothing>"
 reveal_type(C(1))  # N: Revealed type is "__main__.C"
-reveal_type(D())  # N: Revealed type is "<nothing>"
+
+if object():
+    reveal_type(D())  # N: Revealed type is "<nothing>"
 reveal_type(D(1))  # N: Revealed type is "__main__.D"
 
 [case testClassScopeImportWithWrapperAndError]

test-data/unit/check-fastparse.test

@@ -228,8 +228,8 @@ def g():  # E: Type signature has too many arguments
 assert 1, 2
 assert (1, 2)  # E: Assertion is always true, perhaps remove parentheses?
 assert (1, 2), 3  # E: Assertion is always true, perhaps remove parentheses?
-assert ()
 assert (1,)  # E: Assertion is always true, perhaps remove parentheses?
+assert ()
 [builtins fixtures/tuple.pyi]
 
 [case testFastParseAssertMessage]

test-data/unit/check-incremental.test

@@ -5413,7 +5413,8 @@ reveal_type(z)
 [out]
 tmp/c.py:2: note: Revealed type is "a.<subclass of "A" and "B">"
 [out2]
-tmp/c.py:2: note: Revealed type is "a.A"
+tmp/b.py:2: error: Cannot determine type of "y"
+tmp/c.py:2: note: Revealed type is "Any"
 
 [case testIsInstanceAdHocIntersectionIncrementalUnreachaableToIntersection]
 import c
@@ -5444,7 +5445,8 @@ from b import z
 reveal_type(z)
 [builtins fixtures/isinstance.pyi]
 [out]
-tmp/c.py:2: note: Revealed type is "a.A"
+tmp/b.py:2: error: Cannot determine type of "y"
+tmp/c.py:2: note: Revealed type is "Any"
 [out2]
 tmp/c.py:2: note: Revealed type is "a.<subclass of "A" and "B">"
 

test-data/unit/check-inference-context.test

@@ -625,8 +625,10 @@ reveal_type((lambda x, y: x + y)(1, 2))  # N: Revealed type is "builtins.int"
 reveal_type((lambda s, i: s)(i=0, s='x')) # N: Revealed type is "Literal['x']?"
 reveal_type((lambda s, i: i)(i=0, s='x')) # N: Revealed type is "Literal[0]?"
 reveal_type((lambda x, s, i: x)(1.0, i=0, s='x')) # N: Revealed type is "builtins.float"
-(lambda x, s, i: x)() # E: Too few arguments
-(lambda: 0)(1) # E: Too many arguments
+if object():
+    (lambda x, s, i: x)() # E: Too few arguments
+if object():
+    (lambda: 0)(1) # E: Too many arguments
 -- varargs are not handled, but it should not crash
 reveal_type((lambda *k, s, i: i)(type, i=0, s='x')) # N: Revealed type is "Any"
 reveal_type((lambda s, *k, i: i)(i=0, s='x')) # N: Revealed type is "Any"

test-data/unit/check-native-int.test

@@ -87,8 +87,10 @@ reveal_type(meet(f32, f))  # N: Revealed type is "mypy_extensions.i32"
 reveal_type(meet(f, f32))  # N: Revealed type is "mypy_extensions.i32"
 reveal_type(meet(f64, f))  # N: Revealed type is "mypy_extensions.i64"
 reveal_type(meet(f, f64))  # N: Revealed type is "mypy_extensions.i64"
-reveal_type(meet(f32, f64))  # N: Revealed type is "<nothing>"
-reveal_type(meet(f64, f32))  # N: Revealed type is "<nothing>"
+if object():
+    reveal_type(meet(f32, f64))  # N: Revealed type is "<nothing>"
+if object():
+    reveal_type(meet(f64, f32))  # N: Revealed type is "<nothing>"
 
 reveal_type(meet(f, fa))    # N: Revealed type is "builtins.int"
 reveal_type(meet(f32, fa))  # N: Revealed type is "mypy_extensions.i32"
@@ -148,8 +150,10 @@ def meet(c1: Callable[[T], None], c2: Callable[[T], None]) -> T:
 def ff(x: float) -> None: pass
 def fi32(x: i32) -> None: pass
 
-reveal_type(meet(ff, fi32))  # N: Revealed type is "<nothing>"
-reveal_type(meet(fi32, ff))  # N: Revealed type is "<nothing>"
+if object():
+    reveal_type(meet(ff, fi32))  # N: Revealed type is "<nothing>"
+if object():
+    reveal_type(meet(fi32, ff))  # N: Revealed type is "<nothing>"
 [builtins fixtures/dict.pyi]
 
 [case testNativeIntForLoopRange]

test-data/unit/check-statements.test

@@ -409,11 +409,16 @@ main:5: error: Exception must be derived from BaseException
 class A: pass
 class MyError(BaseException): pass
 def f(): pass
-raise BaseException
-raise MyError
-raise A # E: Exception must be derived from BaseException
-raise object # E: Exception must be derived from BaseException
-raise f # E: Exception must be derived from BaseException
+if object():
+    raise BaseException
+if object():
+    raise MyError
+if object():
+    raise A # E: Exception must be derived from BaseException
+if object():
+    raise object # E: Exception must be derived from BaseException
+if object():
+    raise f # E: Exception must be derived from BaseException
 [builtins fixtures/exception.pyi]
 
 [case testRaiseClassObjectCustomInit]
@@ -429,18 +434,30 @@ class MyKwError(Exception):
 class MyErrorWithDefault(Exception):
     def __init__(self, optional=1) -> None:
         ...
-raise BaseException
-raise Exception
-raise BaseException(1)
-raise Exception(2)
-raise MyBaseError(4)
-raise MyError(5, 6)
-raise MyKwError(kwonly=7)
-raise MyErrorWithDefault(8)
-raise MyErrorWithDefault
-raise MyBaseError  # E: Too few arguments for "MyBaseError"
-raise MyError  # E: Too few arguments for "MyError"
-raise MyKwError  # E: Missing named argument "kwonly" for "MyKwError"
+if object():
+    raise BaseException
+if object():
+    raise Exception
+if object():
+    raise BaseException(1)
+if object():
+    raise Exception(2)
+if object():
+    raise MyBaseError(4)
+if object():
+    raise MyError(5, 6)
+if object():
+    raise MyKwError(kwonly=7)
+if object():
+    raise MyErrorWithDefault(8)
+if object():
+    raise MyErrorWithDefault
+if object():
+    raise MyBaseError  # E: Too few arguments for "MyBaseError"
+if object():
+    raise MyError  # E: Too few arguments for "MyError"
+if object():
+    raise MyKwError  # E: Missing named argument "kwonly" for "MyKwError"
 [builtins fixtures/exception.pyi]
 
 [case testRaiseExceptionType]
@@ -473,10 +490,14 @@ f: MyError
 a: A
 x: BaseException
 del x
-raise e from a # E: Exception must be derived from BaseException
-raise e from e
-raise e from f
-raise e from x # E: Trying to read deleted variable "x"
+if object():
+    raise e from a # E: Exception must be derived from BaseException
+if object():
+    raise e from e
+if object():
+    raise e from f
+if object():
+    raise e from x # E: Trying to read deleted variable "x"
 class A: pass
 class MyError(BaseException): pass
 [builtins fixtures/exception.pyi]
@@ -486,11 +507,16 @@ import typing
 class A: pass
 class MyError(BaseException): pass
 def f(): pass
-raise BaseException from BaseException
-raise BaseException from MyError
-raise BaseException from A # E: Exception must be derived from BaseException
-raise BaseException from object # E: Exception must be derived from BaseException
-raise BaseException from f # E: Exception must be derived from BaseException
+if object():
+    raise BaseException from BaseException
+if object():
+    raise BaseException from MyError
+if object():
+    raise BaseException from A # E: Exception must be derived from BaseException
+if object():
+    raise BaseException from object # E: Exception must be derived from BaseException
+if object():
+    raise BaseException from f # E: Exception must be derived from BaseException
 [builtins fixtures/exception.pyi]
 
 [case testTryFinallyStatement]

test-data/unit/check-typevar-tuple.test

@@ -17,7 +17,8 @@ reveal_type(f(args))  # N: Revealed type is "Tuple[builtins.int, builtins.str]"
 
 reveal_type(f(varargs))  # N: Revealed type is "builtins.tuple[builtins.int, ...]"
 
-f(0)  # E: Argument 1 to "f" has incompatible type "int"; expected <nothing>
+if object():
+    f(0)  # E: Argument 1 to "f" has incompatible type "int"; expected <nothing>
 
 def g(a: Tuple[Unpack[Ts]], b: Tuple[Unpack[Ts]]) -> Tuple[Unpack[Ts]]:
     return a

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

@@ -873,15 +873,15 @@ def expect_str(x: str) -> str: pass
 x: int
 if False:
     assert False
-    reveal_type(x)
+    reveal_type(x)  # E: Statement is unreachable
 
 if False:
     raise Exception()
-    reveal_type(x)
+    reveal_type(x)  # E: Statement is unreachable
 
 if False:
     assert_never(x)
-    reveal_type(x)
+    reveal_type(x)  # E: Statement is unreachable
 
 if False:
     nonthrowing_assert_never(x)  # E: Statement is unreachable
@@ -890,7 +890,7 @@ if False:
 if False:
     # Ignore obvious type errors
     assert_never(expect_str(x))
-    reveal_type(x)
+    reveal_type(x)  # E: Statement is unreachable
 [builtins fixtures/exception.pyi]
 
 [case testNeverVariants]

test-data/unit/fine-grained.test

@@ -9667,7 +9667,8 @@ reveal_type(z)
 [out]
 c.py:2: note: Revealed type is "a.<subclass of "A" and "B">"
 ==
-c.py:2: note: Revealed type is "a.A"
+c.py:2: note: Revealed type is "Any"
+b.py:2: error: Cannot determine type of "y"
 
 [case testIsInstanceAdHocIntersectionFineGrainedIncrementalUnreachaableToIntersection]
 import c
@@ -9698,7 +9699,8 @@ from b import z
 reveal_type(z)
 [builtins fixtures/isinstance.pyi]
 [out]
-c.py:2: note: Revealed type is "a.A"
+b.py:2: error: Cannot determine type of "y"
+c.py:2: note: Revealed type is "Any"
 ==
 c.py:2: note: Revealed type is "a.<subclass of "A" and "B">"