Remove date/date comparison warnings as inheritance blocking makes them redundant

Co-authored-by: m-aciek <9288014+m-aciek@users.noreply.github.com>

Author: Copilot

mypy/checkexpr.py

@@ -3886,11 +3886,6 @@ def has_unsafe_subtype_relationship(self, left: Type, right: Type) -> bool:
 
         Returns True if they are instances with a nominal subclass relationship
         that is known to be unsafe (e.g., datetime and date).
-        
-        This also flags comparisons between two values of the supertype (e.g., date vs date)
-        because at runtime, one could be an instance of the unsafe subclass (e.g., datetime).
-        While conservative, this is necessary to catch LSP violations where the subclass
-        has incompatible comparison operators.
         """
         from mypy.subtypes import UNSAFE_SUBTYPING_PAIRS
 
@@ -3909,13 +3904,6 @@ def has_unsafe_subtype_relationship(self, left: Type, right: Type) -> bool:
             if ((left_name == subclass and right_name == superclass)
                     or (left_name == superclass and right_name == subclass)):
                 return True
-            
-            # Also flag when both types are the supertype, as one could be the subclass at runtime.
-            # Example: comparing 'date' vs 'date' is unsafe because one could be a 'datetime'.
-            # This is conservative but necessary due to Python's subtyping allowing datetime
-            # instances to be used wherever date is expected, despite incompatible comparison.
-            if left_name == superclass and right_name == superclass:
-                return True
 
         return False
 

test-data/unit/check-unsafe-subtype.test

@@ -119,8 +119,8 @@ d2: date
 if dt1 < dt2:
     pass
 
-# date vs date is unsafe because one could be a datetime at runtime
-if d1 < d2:  # E: Unsafe comparison between "date" and "date"; runtime comparison may raise TypeError
+# date vs date is now OK since inheritance blocking prevents datetime from being passed as date
+if d1 < d2:
     pass
 [builtins fixtures/classmethod.pyi]
 [file datetime.pyi]
@@ -207,8 +207,8 @@ class datetime(date):
 from datetime import date, datetime
 
 def compare_dates(d1: date, d2: date) -> bool:
-    # This is flagged because one of the dates could be a datetime at runtime
-    return d1 < d2  # E: Unsafe comparison between "date" and "date"; runtime comparison may raise TypeError
+    # With inheritance blocking, this is now safe - datetime cannot be passed here
+    return d1 < d2
 
 # Example usage that would fail at runtime:
 dt = datetime.now()