Teach mypy about NoReturn

mypy/checker.py

@@ -648,7 +648,11 @@ def is_implicit_any(t: Type) -> bool:
                 if self.is_trivial_body(defn.body):
                     pass
                 else:
-                    self.msg.note(messages.MISSING_RETURN_STATEMENT, defn)
+                    if isinstance(self.return_types[-1], UninhabitedType):
+                        # This is a NoReturn function
+                        self.msg.note(messages.INVALID_IMPLICIT_RETURN, defn)
+                    else:
+                        self.msg.note(messages.MISSING_RETURN_STATEMENT, defn)
 
             self.return_types.pop()
 
@@ -1721,6 +1725,10 @@ def check_return_stmt(self, s: ReturnStmt) -> None:
             else:
                 return_type = self.return_types[-1]
 
+            if isinstance(return_type, UninhabitedType):
+                self.fail(messages.NO_RETURN_EXPECTED, s)
+                return
+
             if s.expr:
                 # Return with a value.
                 typ = self.accept(s.expr, return_type)

mypy/checkexpr.py

@@ -184,7 +184,10 @@ def visit_call_expr(self, e: CallExpr) -> Type:
                 isinstance(callee_type, CallableType)
                 and callee_type.implicit):
             return self.msg.untyped_function_call(callee_type, e)
-        return self.check_call_expr_with_callee_type(callee_type, e)
+        ret_type = self.check_call_expr_with_callee_type(callee_type, e)
+        if isinstance(ret_type, UninhabitedType):
+            self.chk.binder.unreachable()
+        return ret_type
 
     def check_typeddict_call(self, callee: TypedDictType,
                              arg_kinds: List[int],

mypy/messages.py

@@ -11,7 +11,8 @@
 from mypy.errors import Errors
 from mypy.types import (
     Type, CallableType, Instance, TypeVarType, TupleType, TypedDictType,
-    UnionType, Void, NoneTyp, AnyType, Overloaded, FunctionLike, DeletedType, TypeType
+    UnionType, Void, NoneTyp, AnyType, Overloaded, FunctionLike, DeletedType, TypeType,
+    UninhabitedType
 )
 from mypy.nodes import (
     TypeInfo, Context, MypyFile, op_methods, FuncDef, reverse_type_aliases,
@@ -24,8 +25,10 @@
 
 NO_RETURN_VALUE_EXPECTED = 'No return value expected'
 MISSING_RETURN_STATEMENT = 'Missing return statement'
+INVALID_IMPLICIT_RETURN = 'Implicit return in function which does not return'
 INCOMPATIBLE_RETURN_VALUE_TYPE = 'Incompatible return value type'
 RETURN_VALUE_EXPECTED = 'Return value expected'
+NO_RETURN_EXPECTED = 'Return statement in function which does not return'
 INVALID_EXCEPTION = 'Exception must be derived from BaseException'
 INVALID_EXCEPTION_TYPE = 'Exception type must be derived from BaseException'
 INVALID_RETURN_TYPE_FOR_GENERATOR = \
@@ -319,6 +322,11 @@ def format_simple(self, typ: Type, verbosity: int = 0) -> str:
             return '"Any"'
         elif isinstance(typ, DeletedType):
             return '<deleted>'
+        elif isinstance(typ, UninhabitedType):
+            if typ.is_noreturn:
+                return 'NoReturn'
+            else:
+                return '<uninhabited>'
         elif isinstance(typ, TypeType):
             return 'Type[{}]'.format(
                 strip_quotes(self.format_simple(typ.item, verbosity)))

mypy/typeanal.py

@@ -148,6 +148,8 @@ def visit_unbound_type(self, t: UnboundType) -> Type:
                 items = self.anal_array(t.args)
                 item = items[0]
                 return TypeType(item, line=t.line)
+            elif fullname == 'mypy_extensions.NoReturn':
+                return UninhabitedType(is_noreturn=True)
             elif sym.kind == TYPE_ALIAS:
                 override = sym.type_override
                 an_args = self.anal_array(t.args)

mypy/types.py

@@ -323,20 +323,23 @@ class UninhabitedType(Type):
 
     can_be_true = False
     can_be_false = False
+    is_noreturn = False  # Does this come from a NoReturn?  Purely for error messages.
 
-    def __init__(self, line: int = -1, column: int = -1) -> None:
+    def __init__(self, is_noreturn: bool = False, line: int = -1, column: int = -1) -> None:
         super().__init__(line, column)
+        self.is_noreturn = is_noreturn
 
     def accept(self, visitor: 'TypeVisitor[T]') -> T:
         return visitor.visit_uninhabited_type(self)
 
     def serialize(self) -> JsonDict:
-        return {'.class': 'UninhabitedType'}
+        return {'.class': 'UninhabitedType',
+                'is_noreturn': self.is_noreturn}
 
     @classmethod
     def deserialize(cls, data: JsonDict) -> 'UninhabitedType':
         assert data['.class'] == 'UninhabitedType'
-        return UninhabitedType()
+        return UninhabitedType(is_noreturn=data['is_noreturn'])
 
 
 class NoneTyp(Type):

test-data/unit/check-flags.test

@@ -136,6 +136,63 @@ def f() -> int:
         return 0
 [builtins fixtures/bool.pyi]
 
+[case testNoReturnDisallowsReturn]
+# flags: --warn-no-return
+from mypy_extensions import NoReturn
+
+def f() -> NoReturn:
+  if bool():
+    return 5  # E: Return statement in function which does not return
+  else:
+    return  # E: Return statement in function which does not return
+[builtins fixtures/dict.pyi]
+
+[case testNoReturnWithoutImplicitReturn]
+# flags: --warn-no-return
+from mypy_extensions import NoReturn
+
+def no_return() -> NoReturn: pass
+def f() -> NoReturn:
+  no_return()
+[builtins fixtures/dict.pyi]
+
+[case testNoReturnDisallowsImplicitReturn]
+# flags: --warn-no-return
+from mypy_extensions import NoReturn
+
+def f() -> NoReturn:  # N: Implicit return in function which does not return
+  non_trivial_function = 1
+[builtins fixtures/dict.pyi]
+
+[case testNoReturnNoWarnNoReturn]
+# flags: --warn-no-return
+from mypy_extensions import NoReturn
+
+def no_return() -> NoReturn: pass
+def f() -> int:
+  if bool():
+    return 0
+  else:
+    no_return()
+[builtins fixtures/dict.pyi]
+
+[case testNoReturnInExpr]
+# flags: --warn-no-return
+from mypy_extensions import NoReturn
+
+def no_return() -> NoReturn: pass
+def f() -> int:
+  return 0
+reveal_type(f() or no_return())  # E: Revealed type is 'builtins.int'
+[builtins fixtures/dict.pyi]
+
+[case testNoReturnVariable]
+# flags: --warn-no-return
+from mypy_extensions import NoReturn
+
+x = 0  # type: NoReturn  # E: Incompatible types in assignment (expression has type "int", variable has type NoReturn)
+[builtins fixtures/dict.pyi]
+
 [case testShowErrorContextFunction]
 # flags: --show-error-context
 def f() -> None:

test-data/unit/fixtures/dict.pyi

@@ -33,3 +33,4 @@ class list(Iterable[T], Generic[T]): # needed by some test cases
 class tuple: pass
 class function: pass
 class float: pass
+class bool: pass

test-data/unit/lib-stub/mypy_extensions.pyi

@@ -4,3 +4,5 @@ T = TypeVar('T')
 
 
 def TypedDict(typename: str, fields: Dict[str, Type[T]]) -> Type[dict]: pass
+
+class NoReturn: pass