feature: new error message for decorator incompatible type

Author: LeoLomardo

mypy/messages.py

@@ -45,6 +45,7 @@
     ClassDef,
     Context,
     Expression,
+    Decorator,
     FuncDef,
     IndexExpr,
     MypyFile,
@@ -772,13 +773,27 @@ def incompatible_argument(
                 actual_type_str, expected_type_str
             )
         else:
-            if self.prefer_simple_messages():
+            try:
+                expected_type = callee.arg_types[m - 1]
+            except IndexError:  # Varargs callees
+                expected_type = callee.arg_types[-1]
+
+            decorator_context = callee_name is None and isinstance(outer_context, Decorator)
+            simple_message = self.prefer_simple_messages() and not decorator_context
+
+            if decorator_context:
+                arg_type_str, expected_type_str = format_type_distinctly(
+                    arg_type, expected_type, bare=True, options=self.options
+                )
+                func_name = outer_context.func.name
+                msg = (
+                    f'Decorated function "{func_name}" has incompatible type '
+                    f'{quote_type_string(arg_type_str)}; expected '
+                    f'{quote_type_string(expected_type_str)}'
+                )
+            elif simple_message:
                 msg = "Argument has incompatible type"
             else:
-                try:
-                    expected_type = callee.arg_types[m - 1]
-                except IndexError:  # Varargs callees
-                    expected_type = callee.arg_types[-1]
                 arg_type_str, expected_type_str = format_type_distinctly(
                     arg_type, expected_type, bare=True, options=self.options
                 )
@@ -822,6 +837,7 @@ def incompatible_argument(
                         quote_type_string(arg_type_str),
                         quote_type_string(expected_type_str),
                     )
+            if not simple_message:
                 expected_type = get_proper_type(expected_type)
                 if isinstance(expected_type, UnionType):
                     expected_types = list(expected_type.items)

test-data/unit/check-functions.test

@@ -959,6 +959,16 @@ def dec2(f: Callable[[Any, Any], None]) -> Callable[[Any], None]: pass
 @dec2
 def f(x, y): pass
 
+[case testDecoratorFactoryApplicationErrorMessage]
+from typing import Callable
+
+def decorator(f: object) -> Callable[[Callable[[int], object]], None]: ...
+def f(a: int) -> None: ...
+
+@decorator(f)  # E: Decorated function "something" has incompatible type "Callable[[], None]"; expected "Callable[[int], object]"
+def something() -> None:
+    pass
+
 [case testNoTypeCheckDecoratorOnMethod1]
 from typing import no_type_check