More helpful error for missing self (#14386)

Fixes #14385

Author: hauntsaninja

mypy/semanal.py

@@ -947,7 +947,10 @@ def prepare_method_signature(self, func: FuncDef, info: TypeInfo, has_self_type:
             if func.name in ["__init_subclass__", "__class_getitem__"]:
                 func.is_class = True
             if not func.arguments:
-                self.fail("Method must have at least one argument", func)
+                self.fail(
+                    'Method must have at least one argument. Did you forget the "self" argument?',
+                    func,
+                )
             elif isinstance(functype, CallableType):
                 self_type = get_proper_type(functype.arg_types[0])
                 if isinstance(self_type, AnyType):

test-data/unit/check-classes.test

@@ -2901,7 +2901,7 @@ b.bad = 'a'  # E: Incompatible types in assignment (expression has type "str", v
 from typing import Any
 
 class Test:
-    def __setattr__() -> None: ...  # E: Method must have at least one argument # E: Invalid signature "Callable[[], None]" for "__setattr__"
+    def __setattr__() -> None: ...  # E: Method must have at least one argument. Did you forget the "self" argument? # E: Invalid signature "Callable[[], None]" for "__setattr__"
 t = Test()
 t.crash = 'test'  # E: "Test" has no attribute "crash"
 
@@ -7120,7 +7120,7 @@ reveal_type(Foo().y)  # N: Revealed type is "builtins.list[Any]"
 # flags: --check-untyped-defs
 
 class Foo:
-    def bad():  # E: Method must have at least one argument
+    def bad():  # E: Method must have at least one argument. Did you forget the "self" argument?
         self.x = 0  # E: Name "self" is not defined
 
 [case testTypeAfterAttributeAccessWithDisallowAnyExpr]

test-data/unit/check-functions.test

@@ -2708,7 +2708,7 @@ class A:
     @dec
     def e(self) -> int: pass
     @property
-    def g() -> int: pass   # E: Method must have at least one argument
+    def g() -> int: pass   # E: Method must have at least one argument. Did you forget the "self" argument?
     @property
     def h(self, *args, **kwargs) -> int: pass   # OK
 [builtins fixtures/property.pyi]

test-data/unit/check-super.test

@@ -365,7 +365,7 @@ class A:
     def f(self) -> None: pass
 
 class B(A):
-    def g() -> None: # E: Method must have at least one argument
+    def g() -> None: # E: Method must have at least one argument. Did you forget the "self" argument?
         super().f() # E: super() requires one or more positional arguments in enclosing function
     def h(self) -> None:
         def a() -> None:

test-data/unit/fine-grained.test

@@ -1586,11 +1586,11 @@ class A:
 [file b.py.3]
 2
 [out]
-a.py:3: error: Method must have at least one argument
+a.py:3: error: Method must have at least one argument. Did you forget the "self" argument?
 ==
-a.py:3: error: Method must have at least one argument
+a.py:3: error: Method must have at least one argument. Did you forget the "self" argument?
 ==
-a.py:3: error: Method must have at least one argument
+a.py:3: error: Method must have at least one argument. Did you forget the "self" argument?
 
 [case testBaseClassDeleted]
 import m
@@ -2007,11 +2007,11 @@ class A:
 class A:
     def foo(self) -> int: pass
 [out]
-a.py:2: error: Method must have at least one argument
+a.py:2: error: Method must have at least one argument. Did you forget the "self" argument?
 ==
-a.py:2: error: Method must have at least one argument
+a.py:2: error: Method must have at least one argument. Did you forget the "self" argument?
 ==
-a.py:2: error: Method must have at least one argument
+a.py:2: error: Method must have at least one argument. Did you forget the "self" argument?
 ==
 
 [case testPreviousErrorInMethodSemanal2]

test-data/unit/semanal-errors.test

@@ -542,7 +542,7 @@ import typing
 class A:
   def f(): pass
 [out]
-main:3: error: Method must have at least one argument
+main:3: error: Method must have at least one argument. Did you forget the "self" argument?
 
 [case testInvalidBaseClass]
 import typing
@@ -564,8 +564,8 @@ class A:
   def f() -> None: pass
   def g(): pass
 [out]
-main:3: error: Method must have at least one argument
-main:4: error: Method must have at least one argument
+main:3: error: Method must have at least one argument. Did you forget the "self" argument?
+main:4: error: Method must have at least one argument. Did you forget the "self" argument?
 
 [case testMultipleMethodDefinition]
 import typing