Make notes default to None code

mypy/checker.py

@@ -7734,7 +7734,7 @@ def fail(
     ) -> ErrorInfo:
         """Produce an error message."""
         if isinstance(msg, ErrorMessage):
-            return self.msg.fail(msg.value, context, code=msg.code)
+            msg, code = msg.value, msg.code  # type: ignore[attr-defined] #TODO: https://github.com/python/mypy/issues/19891
         return self.msg.fail(msg, context, code=code)
 
     def note(
@@ -7744,12 +7744,12 @@ def note(
         offset: int = 0,
         *,
         code: ErrorCode | None = None,
+        parent_error: ErrorInfo | None = None,
     ) -> None:
         """Produce a note."""
         if isinstance(msg, ErrorMessage):
-            self.msg.note(msg.value, context, code=msg.code)
-            return
-        self.msg.note(msg, context, offset=offset, code=code)
+            msg, code = msg.value, msg.code  # type: ignore[attr-defined] #TODO: https://github.com/python/mypy/issues/19891
+        self.msg.note(msg, context, offset=offset, code=code, parent_error=parent_error)
 
     def iterable_item_type(
         self, it: Instance | CallableType | TypeType | Overloaded, context: Context

mypy/errors.py

@@ -509,7 +509,7 @@ def report(
             line: line number of error
             column: column number of error
             message: message to report
-            code: error code (defaults to 'misc'; not shown for notes)
+            code: error code (defaults to misc; or None for notes), not shown for notes
             blocker: if True, don't continue analysis after this error
             severity: 'error' or 'note'
             file: if non-None, override current file as context
@@ -548,7 +548,16 @@ def report(
             end_line = line
 
         code = code or (parent_error.code if parent_error else None)
-        code = code or (codes.MISC if not blocker else None)
+        if code is None:
+            if blocker:
+                code = None  # do we even need to do this for blockers?
+            elif severity == "note":
+                if parent_error is not None:
+                    code = parent_error.code  # this might be None, btw
+                else:
+                    code = None
+            else:
+                code = codes.MISC
 
         info = ErrorInfo(
             import_ctx=self.import_context(),

mypy/messages.py

@@ -1516,9 +1516,9 @@ def incompatible_conditional_function_def(
         if isinstance(old_type, (CallableType, Overloaded)) and isinstance(
             new_type, (CallableType, Overloaded)
         ):
-            self.note("Original:", defn)
+            self.note("Original:", defn, parent_error=error)
             self.pretty_callable_or_overload(old_type, defn, offset=4, parent_error=error)
-            self.note("Redefinition:", defn)
+            self.note("Redefinition:", defn, parent_error=error)
             self.pretty_callable_or_overload(new_type, defn, offset=4, parent_error=error)
 
     def cannot_instantiate_abstract_class(

mypy/plugins/proper_plugin.py

@@ -63,7 +63,7 @@ def isinstance_proper_hook(ctx: FunctionContext) -> Type:
         ) and is_dangerous_target(right):
             if is_special_target(right):
                 return ctx.default_return_type
-            ctx.api.fail(
+            parent_error = ctx.api.fail(
                 "Never apply isinstance() to unexpanded types;"
                 " use mypy.types.get_proper_type() first",
                 ctx.context,
@@ -72,6 +72,7 @@ def isinstance_proper_hook(ctx: FunctionContext) -> Type:
                 "If you pass on the original type"
                 " after the check, always use its unexpanded version",
                 ctx.context,
+                parent_error=parent_error,
             )
     return ctx.default_return_type
 

test-data/unit/check-errorcodes.test

@@ -1,6 +1,27 @@
 -- Tests for error codes and ignoring errors using error codes
 --
--- These implicitly use --show-error-codes.
+-- These implicitly use --show-error-codes. # TODO: isn't this statement redundant now that this flag defaults to true?
+
+[case testErrorCodeMiscDoesntSuppressRevealType]
+# Prevent regression on https://github.com/python/mypy/issues/19840
+from typing import reveal_type
+reveal_type(1) #type: ignore[misc] # N: Revealed type is "Literal[1]?"
+
+[case testErrorCodeMiscDoesntSuppressRevealLocals]
+reveal_locals() #type:ignore[misc] # N: There are no locals to reveal
+
+[case testErrorCodeMiscSuppressesAccompanyingNotes]
+# flags: --strict
+if x: #type: ignore[name-defined]
+    def y() -> None: pass
+else:
+    def y() -> bool: return True # type: ignore[misc]
+    def y() -> str: return "hmm" # All conditional function variants must have identical signatures  [misc] # E: All conditional function variants must have identical signatures  [misc] \
+                                                                                                            # N: Original: \
+                                                                                                            # N:     def y() -> None \
+                                                                                                            # N: Redefinition: \
+                                                                                                            # N:     def y() -> str
+
 
 [case testErrorCodeNoAttribute]
 import m