Handle edge case

Author: cdce8p

mypy/checkpattern.py

@@ -344,7 +344,9 @@ def visit_sequence_pattern(self, o: SequencePattern) -> PatternType:
             contracted_rest_inner_types.append(rest)
             self.update_type_map(captures, type_map)
             if s:
-                self.update_type_map(captures, {subject: typ for subject in s})
+                self.update_type_map(
+                    captures, {subject: typ for subject in s}, fail_multiple_assignments=False
+                )
 
         new_inner_types = self.expand_starred_pattern_types(
             contracted_new_inner_types, star_position, len(inner_types), unpack_index is not None
@@ -803,7 +805,10 @@ def generate_types_from_names(self, type_names: list[str]) -> list[Type]:
         return types
 
     def update_type_map(
-        self, original_type_map: dict[Expression, Type], extra_type_map: dict[Expression, Type]
+        self,
+        original_type_map: dict[Expression, Type],
+        extra_type_map: dict[Expression, Type],
+        fail_multiple_assignments: bool = True,
     ) -> None:
         # Calculating this would not be needed if TypeMap directly used literal hashes instead of
         # expressions, as suggested in the TODO above it's definition
@@ -812,9 +817,10 @@ def update_type_map(
             if literal_hash(expr) in already_captured:
                 if (node := get_var(expr)) is None:
                     continue
-                self.msg.fail(
-                    message_registry.MULTIPLE_ASSIGNMENTS_IN_PATTERN.format(node.name), expr
-                )
+                if fail_multiple_assignments:
+                    self.msg.fail(
+                        message_registry.MULTIPLE_ASSIGNMENTS_IN_PATTERN.format(node.name), expr
+                    )
             else:
                 original_type_map[expr] = typ
 

test-data/unit/check-python310.test

@@ -3074,6 +3074,10 @@ match (m, n, o):
     case [1, *_, 3, 4, 5] | [2, *_, 3, 4, 5]:
         # No match -> don't crash
         reveal_type(m)  # E: Statement is unreachable
+    case [m, *_]:
+        # This will always match and bind the variables to itself
+        # Although it doesn't make much sense, make sure it doesn't raise an error
+        reveal_type(m)  # N: Revealed type is "builtins.object"
 
 match [m, n, o]:
     case [1, 2, 3] | [2, 3, 4]: