Support ==-based narrowing of Optional (#18163)

Closes #18135

This change implements the third approach mentioned in #18135, which is
stricter than similar narrowings, as clarified by the new/modified code
comments. Personally, I prefer this more stringent way but could also
switch this PR to approach two if there is consent that convenience is
more important than type safety here.

mypy/checker.py

@@ -6274,10 +6274,6 @@ def has_no_custom_eq_checks(t: Type) -> bool:
                 coerce_only_in_literal_context,
             )
 
-        # Strictly speaking, we should also skip this check if the objects in the expr
-        # chain have custom __eq__ or __ne__ methods. But we (maybe optimistically)
-        # assume nobody would actually create a custom objects that considers itself
-        # equal to None.
         if if_map == {} and else_map == {}:
             if_map, else_map = self.refine_away_none_in_comparison(
                 operands, operand_types, expr_indices, narrowable_operand_index_to_hash.keys()
@@ -6602,25 +6598,36 @@ def refine_away_none_in_comparison(
         For more details about what the different arguments mean, see the
         docstring of 'refine_identity_comparison_expression' up above.
         """
+
         non_optional_types = []
         for i in chain_indices:
             typ = operand_types[i]
             if not is_overlapping_none(typ):
                 non_optional_types.append(typ)
 
-        # Make sure we have a mixture of optional and non-optional types.
-        if len(non_optional_types) == 0 or len(non_optional_types) == len(chain_indices):
-            return {}, {}
+        if_map, else_map = {}, {}
 
-        if_map = {}
-        for i in narrowable_operand_indices:
-            expr_type = operand_types[i]
-            if not is_overlapping_none(expr_type):
-                continue
-            if any(is_overlapping_erased_types(expr_type, t) for t in non_optional_types):
-                if_map[operands[i]] = remove_optional(expr_type)
+        if not non_optional_types or (len(non_optional_types) != len(chain_indices)):
 
-        return if_map, {}
+            # Narrow e.g. `Optional[A] == "x"` or `Optional[A] is "x"` to `A` (which may be
+            # convenient but is strictly not type-safe):
+            for i in narrowable_operand_indices:
+                expr_type = operand_types[i]
+                if not is_overlapping_none(expr_type):
+                    continue
+                if any(is_overlapping_erased_types(expr_type, t) for t in non_optional_types):
+                    if_map[operands[i]] = remove_optional(expr_type)
+
+            # Narrow e.g. `Optional[A] != None` to `A` (which is stricter than the above step and
+            # so type-safe but less convenient, because e.g. `Optional[A] == None` still results
+            # in `Optional[A]`):
+            if any(isinstance(get_proper_type(ot), NoneType) for ot in operand_types):
+                for i in narrowable_operand_indices:
+                    expr_type = operand_types[i]
+                    if is_overlapping_none(expr_type):
+                        else_map[operands[i]] = remove_optional(expr_type)
+
+        return if_map, else_map
 
     def is_len_of_tuple(self, expr: Expression) -> bool:
         """Is this expression a `len(x)` call where x is a tuple or union of tuples?"""

test-data/unit/check-narrowing.test

@@ -1385,9 +1385,9 @@ val: Optional[A]
 if val == None:
     reveal_type(val)  # N: Revealed type is "Union[__main__.A, None]"
 else:
-    reveal_type(val)  # N: Revealed type is "Union[__main__.A, None]"
+    reveal_type(val)  # N: Revealed type is "__main__.A"
 if val != None:
-    reveal_type(val)  # N: Revealed type is "Union[__main__.A, None]"
+    reveal_type(val)  # N: Revealed type is "__main__.A"
 else:
     reveal_type(val)  # N: Revealed type is "Union[__main__.A, None]"