diff --git a/mypy/checker.py b/mypy/checker.py
index fe978385d12c..4cad81cd0ca6 100644
--- a/mypy/checker.py
+++ b/mypy/checker.py
@@ -1262,6 +1262,19 @@ def check_default_args(self, item: FuncItem, body_is_trivial: bool) -> None:
                 msg += f"tuple argument {name[12:]}"
             else:
                 msg += f'argument "{name}"'
+            if (
+                not self.options.implicit_optional
+                and isinstance(arg.initializer, NameExpr)
+                and arg.initializer.fullname == "builtins.None"
+            ):
+                notes = [
+                    "PEP 484 prohibits implicit Optional. "
+                    "Accordingly, mypy has changed its default to no_implicit_optional=True",
+                    "Use https://github.com/hauntsaninja/no_implicit_optional to automatically "
+                    "upgrade your codebase",
+                ]
+            else:
+                notes = None
             self.check_simple_assignment(
                 arg.variable.type,
                 arg.initializer,
@@ -1269,6 +1282,7 @@ def check_default_args(self, item: FuncItem, body_is_trivial: bool) -> None:
                 msg=ErrorMessage(msg, code=codes.ASSIGNMENT),
                 lvalue_name="argument",
                 rvalue_name="default",
+                notes=notes,
             )
 
     def is_forward_op_method(self, method_name: str) -> bool:
@@ -3739,6 +3753,8 @@ def check_simple_assignment(
         msg: ErrorMessage = message_registry.INCOMPATIBLE_TYPES_IN_ASSIGNMENT,
         lvalue_name: str = "variable",
         rvalue_name: str = "expression",
+        *,
+        notes: list[str] | None = None,
     ) -> Type:
         if self.is_stub and isinstance(rvalue, EllipsisExpr):
             # '...' is always a valid initializer in a stub.
@@ -3763,6 +3779,7 @@ def check_simple_assignment(
                     msg,
                     f"{rvalue_name} has type",
                     f"{lvalue_name} has type",
+                    notes=notes,
                 )
             return rvalue_type
 
@@ -5666,6 +5683,7 @@ def check_subtype(
         subtype_label: str | None = None,
         supertype_label: str | None = None,
         *,
+        notes: list[str] | None = None,
         code: ErrorCode | None = None,
         outer_context: Context | None = None,
     ) -> bool:
@@ -5681,6 +5699,7 @@ def check_subtype(
         subtype_label: str | None = None,
         supertype_label: str | None = None,
         *,
+        notes: list[str] | None = None,
         outer_context: Context | None = None,
     ) -> bool:
         ...
@@ -5694,6 +5713,7 @@ def check_subtype(
         subtype_label: str | None = None,
         supertype_label: str | None = None,
         *,
+        notes: list[str] | None = None,
         code: ErrorCode | None = None,
         outer_context: Context | None = None,
     ) -> bool:
@@ -5714,7 +5734,7 @@ def check_subtype(
             return False
         extra_info: list[str] = []
         note_msg = ""
-        notes: list[str] = []
+        notes = notes or []
         if subtype_label is not None or supertype_label is not None:
             subtype_str, supertype_str = format_type_distinctly(orig_subtype, orig_supertype)
             if subtype_label is not None:
@@ -5725,7 +5745,7 @@ def check_subtype(
                 outer_context or context, subtype, supertype, supertype_str
             )
             if isinstance(subtype, Instance) and isinstance(supertype, Instance):
-                notes = append_invariance_notes([], subtype, supertype)
+                notes = append_invariance_notes(notes, subtype, supertype)
         if extra_info:
             msg = msg.with_additional_msg(" (" + ", ".join(extra_info) + ")")
 
diff --git a/test-data/unit/check-optional.test b/test-data/unit/check-optional.test
index d40ebf993581..db07290f7b40 100644
--- a/test-data/unit/check-optional.test
+++ b/test-data/unit/check-optional.test
@@ -136,7 +136,9 @@ f(None)
 
 [case testNoInferOptionalFromDefaultNone]
 # flags: --no-implicit-optional
-def f(x: int = None) -> None:  # E: Incompatible default for argument "x" (default has type "None", argument has type "int")
+def f(x: int = None) -> None:  # E: Incompatible default for argument "x" (default has type "None", argument has type "int") \
+                               # N: PEP 484 prohibits implicit Optional. Accordingly, mypy has changed its default to no_implicit_optional=True \
+                               # N: Use https://github.com/hauntsaninja/no_implicit_optional to automatically upgrade your codebase
   pass
 [out]
 
@@ -151,7 +153,9 @@ f(None)
 
 [case testNoInferOptionalFromDefaultNoneComment]
 # flags: --no-implicit-optional
-def f(x=None):  # E: Incompatible default for argument "x" (default has type "None", argument has type "int")
+def f(x=None):  # E: Incompatible default for argument "x" (default has type "None", argument has type "int") \
+                # N: PEP 484 prohibits implicit Optional. Accordingly, mypy has changed its default to no_implicit_optional=True \
+                # N: Use https://github.com/hauntsaninja/no_implicit_optional to automatically upgrade your codebase
   # type: (int) -> None
   pass
 [out]