Fix message when __init__ does not have None return type

Fixes #604.

Author: JukkaL

mypy/checker.py

@@ -502,7 +502,7 @@ def check_func_def(self, defn: FuncItem, typ: CallableType, name: str) -> None:
                 if (fdef.info and fdef.name() == '__init__' and
                         not isinstance(typ.ret_type, Void) and
                         not self.dynamic_funcs[-1]):
-                    self.fail(messages.INIT_MUST_NOT_HAVE_RETURN_TYPE,
+                    self.fail(messages.INIT_MUST_HAVE_NONE_RETURN_TYPE,
                               item.type)
 
             if name in nodes.reverse_op_method_set:

mypy/messages.py

@@ -39,7 +39,7 @@
 INCOMPATIBLE_TYPES_IN_YIELD = 'Incompatible types in yield'
 INCOMPATIBLE_TYPES_IN_YIELD_FROM = 'Incompatible types in "yield from"'
 INCOMPATIBLE_TYPES_IN_STR_INTERPOLATION = 'Incompatible types in string interpolation'
-INIT_MUST_NOT_HAVE_RETURN_TYPE = 'Cannot define return type for "__init__"'
+INIT_MUST_HAVE_NONE_RETURN_TYPE = 'The return type of "__init__" must be None'
 GETTER_TYPE_INCOMPATIBLE_WITH_SETTER = \
     'Type of getter incompatible with setter'
 TUPLE_INDEX_MUST_BE_AN_INT_LITERAL = 'Tuple index must an integer literal'

mypy/test/data/check-classes.test

@@ -367,7 +367,15 @@ class A:
     def __init__(self) -> 'A': pass
 [out]
 main: In member "__init__" of class "A":
-main, line 3: Cannot define return type for "__init__"
+main, line 3: The return type of "__init__" must be None
+
+[case testConstructorWithImplicitReturnValueType]
+import typing
+class A:
+    def __init__(self, x: int): pass
+[out]
+main: In member "__init__" of class "A":
+main, line 3: The return type of "__init__" must be None
 
 [case testGlobalFunctionInitWithReturnType]
 import typing