Improved match subject inference

cdce8p · cdce8p · commit 5f6243523be9 · 2025-08-26T13:38:18.000+02:00
diff --git a/mypy/checker.py b/mypy/checker.py
@@ -5567,7 +5567,9 @@ def visit_match_stmt(self, s: MatchStmt) -> None:
             # capture variable may depend on multiple patterns (it
             # will be a union of all capture types). This pass ignores
             # guard expressions.
-            pattern_types = [self.pattern_checker.accept(p, subject_type) for p in s.patterns]
+            pattern_types = [
+                self.pattern_checker.accept(p, subject_type, [s.subject]) for p in s.patterns
+            ]
             type_maps: list[TypeMap] = [t.captures for t in pattern_types]
             inferred_types = self.infer_variable_types_from_type_maps(type_maps)
 
@@ -5577,7 +5579,7 @@ def visit_match_stmt(self, s: MatchStmt) -> None:
                 current_subject_type = self.expr_checker.narrow_type_from_binder(
                     named_subject, subject_type
                 )
-                pattern_type = self.pattern_checker.accept(p, current_subject_type)
+                pattern_type = self.pattern_checker.accept(p, current_subject_type, [s.subject])
                 with self.binder.frame_context(can_skip=True, fall_through=2):
                     if b.is_unreachable or isinstance(
                         get_proper_type(pattern_type.type), UninhabitedType
diff --git a/mypy/checkpattern.py b/mypy/checkpattern.py
@@ -10,11 +10,23 @@
 from mypy.checkmember import analyze_member_access
 from mypy.expandtype import expand_type_by_instance
 from mypy.join import join_types
-from mypy.literals import literal_hash
+from mypy.literals import Key, literal_hash
 from mypy.maptype import map_instance_to_supertype
 from mypy.meet import narrow_declared_type
 from mypy.messages import MessageBuilder
-from mypy.nodes import ARG_POS, Context, Expression, NameExpr, TypeAlias, TypeInfo, Var
+from mypy.nodes import (
+    ARG_POS,
+    Context,
+    Expression,
+    IndexExpr,
+    IntExpr,
+    MemberExpr,
+    NameExpr,
+    TypeAlias,
+    TypeInfo,
+    UnaryExpr,
+    Var,
+)
 from mypy.options import Options
 from mypy.patterns import (
     AsPattern,
@@ -96,10 +108,8 @@ class PatternChecker(PatternVisitor[PatternType]):
     msg: MessageBuilder
     # Currently unused
     plugin: Plugin
-    # The expression being matched against the pattern
-    subject: Expression
-
-    subject_type: Type
+    # The expressions being matched against the (sub)pattern
+    subject_context: list[list[Expression]]
     # Type of the subject to check the (sub)pattern against
     type_context: list[Type]
     # Types that match against self instead of their __match_args__ if used as a class pattern
@@ -118,24 +128,28 @@ def __init__(
         self.msg = msg
         self.plugin = plugin
 
+        self.subject_context = []
         self.type_context = []
         self.self_match_types = self.generate_types_from_names(self_match_type_names)
         self.non_sequence_match_types = self.generate_types_from_names(
             non_sequence_match_type_names
         )
         self.options = options
 
-    def accept(self, o: Pattern, type_context: Type) -> PatternType:
+    def accept(self, o: Pattern, type_context: Type, subject: list[Expression]) -> PatternType:
+        self.subject_context.append(subject)
         self.type_context.append(type_context)
         result = o.accept(self)
+        self.subject_context.pop()
         self.type_context.pop()
 
         return result
 
     def visit_as_pattern(self, o: AsPattern) -> PatternType:
+        current_subject = self.subject_context[-1]
         current_type = self.type_context[-1]
         if o.pattern is not None:
-            pattern_type = self.accept(o.pattern, current_type)
+            pattern_type = self.accept(o.pattern, current_type, current_subject)
             typ, rest_type, type_map = pattern_type
         else:
             typ, rest_type, type_map = current_type, UninhabitedType(), {}
@@ -150,14 +164,15 @@ def visit_as_pattern(self, o: AsPattern) -> PatternType:
         return PatternType(typ, rest_type, type_map)
 
     def visit_or_pattern(self, o: OrPattern) -> PatternType:
+        current_subject = self.subject_context[-1]
         current_type = self.type_context[-1]
 
         #
         # Check all the subpatterns
         #
-        pattern_types = []
+        pattern_types: list[PatternType] = []
         for pattern in o.patterns:
-            pattern_type = self.accept(pattern, current_type)
+            pattern_type = self.accept(pattern, current_type, current_subject)
             pattern_types.append(pattern_type)
             if not is_uninhabited(pattern_type.type):
                 current_type = pattern_type.rest_type
@@ -173,28 +188,40 @@ def visit_or_pattern(self, o: OrPattern) -> PatternType:
         #
         # Check the capture types
         #
-        capture_types: dict[Var, list[tuple[Expression, Type]]] = defaultdict(list)
+        capture_types: dict[Var, dict[Key | None, list[tuple[Expression, Type]]]] = defaultdict(
+            lambda: defaultdict(list)
+        )
+        capture_expr_keys: set[Key | None] = set()
         # Collect captures from the first subpattern
         for expr, typ in pattern_types[0].captures.items():
             node = get_var(expr)
-            capture_types[node].append((expr, typ))
+            key = literal_hash(expr)
+            capture_types[node][key].append((expr, typ))
+            if isinstance(expr, NameExpr):
+                capture_expr_keys.add(key)
 
         # Check if other subpatterns capture the same names
         for i, pattern_type in enumerate(pattern_types[1:]):
-            vars = {get_var(expr) for expr, _ in pattern_type.captures.items()}
-            if capture_types.keys() != vars:
+            vars = {
+                literal_hash(expr) for expr in pattern_type.captures if isinstance(expr, NameExpr)
+            }
+            if capture_expr_keys != vars:
+                # Only fail for directly captured names (with NameExpr)
                 self.msg.fail(message_registry.OR_PATTERN_ALTERNATIVE_NAMES, o.patterns[i])
             for expr, typ in pattern_type.captures.items():
                 node = get_var(expr)
-                capture_types[node].append((expr, typ))
+                key = literal_hash(expr)
+                capture_types[node][key].append((expr, typ))
 
         captures: dict[Expression, Type] = {}
-        for capture_list in capture_types.values():
-            typ = UninhabitedType()
-            for _, other in capture_list:
-                typ = make_simplified_union([typ, other])
+        for expressions in capture_types.values():
+            for key, capture_list in expressions.items():
+                if other_types := [entry[1] for entry in capture_list]:
+                    typ = make_simplified_union(other_types)
+                else:
+                    typ = UninhabitedType()
 
-            captures[capture_list[0][0]] = typ
+                captures[capture_list[0][0]] = typ
 
         union_type = make_simplified_union(types)
         return PatternType(union_type, current_type, captures)
@@ -284,12 +311,24 @@ def visit_sequence_pattern(self, o: SequencePattern) -> PatternType:
         contracted_inner_types = self.contract_starred_pattern_types(
             inner_types, star_position, required_patterns
         )
-        for p, t in zip(o.patterns, contracted_inner_types):
-            pattern_type = self.accept(p, t)
+        current_subjects: list[list[Expression]] = [[] for _ in range(len(contracted_inner_types))]
+        for s in self.subject_context[-1]:
+            # Support x[0], x[1], ... lookup until wildcard
+            end_pos = len(contracted_inner_types) if star_position is None else star_position
+            for i in range(end_pos):
+                current_subjects[i].append(IndexExpr(s, IntExpr(i)))
+            # For everything after wildcard use x[-2], x[-1]
+            for i in range((star_position or -1) + 1, len(contracted_inner_types)):
+                offset = len(contracted_inner_types) - i
+                current_subjects[i].append(IndexExpr(s, UnaryExpr("-", IntExpr(offset))))
+        for p, t, s in zip(o.patterns, contracted_inner_types, current_subjects):
+            pattern_type = self.accept(p, t, s)
             typ, rest, type_map = pattern_type
             contracted_new_inner_types.append(typ)
             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})
 
         new_inner_types = self.expand_starred_pattern_types(
             contracted_new_inner_types, star_position, len(inner_types), unpack_index is not None
@@ -473,11 +512,18 @@ def visit_mapping_pattern(self, o: MappingPattern) -> PatternType:
             if inner_type is None:
                 can_match = False
                 inner_type = self.chk.named_type("builtins.object")
-            pattern_type = self.accept(value, inner_type)
+            current_subjects: list[Expression] = [
+                IndexExpr(s, key) for s in self.subject_context[-1]
+            ]
+            pattern_type = self.accept(value, inner_type, current_subjects)
             if is_uninhabited(pattern_type.type):
                 can_match = False
             else:
                 self.update_type_map(captures, pattern_type.captures)
+                if current_subjects:
+                    self.update_type_map(
+                        captures, {subject: pattern_type.type for subject in current_subjects}
+                    )
 
         if o.rest is not None:
             mapping = self.chk.named_type("typing.Mapping")
@@ -581,7 +627,7 @@ def visit_class_pattern(self, o: ClassPattern) -> PatternType:
             if self.should_self_match(typ):
                 if len(o.positionals) > 1:
                     self.msg.fail(message_registry.CLASS_PATTERN_TOO_MANY_POSITIONAL_ARGS, o)
-                pattern_type = self.accept(o.positionals[0], narrowed_type)
+                pattern_type = self.accept(o.positionals[0], narrowed_type, [])
                 if not is_uninhabited(pattern_type.type):
                     return PatternType(
                         pattern_type.type,
@@ -681,11 +727,20 @@ def visit_class_pattern(self, o: ClassPattern) -> PatternType:
                 elif keyword is not None:
                     new_type = self.chk.add_any_attribute_to_type(new_type, keyword)
 
-            inner_type, inner_rest_type, inner_captures = self.accept(pattern, key_type)
+            current_subjects: list[Expression] = []
+            if keyword is not None:
+                current_subjects = [MemberExpr(s, keyword) for s in self.subject_context[-1]]
+            inner_type, inner_rest_type, inner_captures = self.accept(
+                pattern, key_type, current_subjects
+            )
             if is_uninhabited(inner_type):
                 can_match = False
             else:
                 self.update_type_map(captures, inner_captures)
+                if current_subjects:
+                    self.update_type_map(
+                        captures, {subject: inner_type for subject in current_subjects}
+                    )
                 if not is_uninhabited(inner_rest_type):
                     rest_type = current_type
 
@@ -799,6 +854,10 @@ def get_var(expr: Expression) -> Var:
     Warning: this in only true for expressions captured by a match statement.
     Don't call it from anywhere else
     """
+    if isinstance(expr, MemberExpr):
+        return get_var(expr.expr)
+    if isinstance(expr, IndexExpr):
+        return get_var(expr.base)
     assert isinstance(expr, NameExpr), expr
     node = expr.node
     assert isinstance(node, Var), node
diff --git a/mypy/literals.py b/mypy/literals.py
@@ -228,7 +228,7 @@ def visit_set_expr(self, e: SetExpr) -> Key | None:
         return self.seq_expr(e, "Set")
 
     def visit_index_expr(self, e: IndexExpr) -> Key | None:
-        if literal(e.index) == LITERAL_YES:
+        if literal(e.index) != LITERAL_NO:
             return ("Index", literal_hash(e.base), literal_hash(e.index))
         return None
 
diff --git a/test-data/unit/check-python310.test b/test-data/unit/check-python310.test
@@ -2965,3 +2965,86 @@ val: int = 8
 match val:
     case FOO:  # E: Cannot assign to final name "FOO"
         pass
+
+[case testMatchSubjectInferenceSequence]
+m: object
+
+match m:
+    case [1, True]:
+        reveal_type(m)  # N: Revealed type is "typing.Sequence[builtins.int]"
+        reveal_type(m[0])  # N: Revealed type is "Literal[1]"
+        reveal_type(m[-2])  # N: Revealed type is "Literal[1]"
+        reveal_type(m[1])  # N: Revealed type is "Literal[True]"
+        reveal_type(m[-1])  # N: Revealed type is "Literal[True]"
+    case [1, *_, False]:
+        reveal_type(m)  # N: Revealed type is "typing.Sequence[builtins.object]"
+        reveal_type(m[-1])  # N: Revealed type is "Literal[False]"
+    case [[1], [True]]:
+        reveal_type(m[0][0])  # N: Revealed type is "Literal[1]"
+        reveal_type(m[-2][0])  # N: Revealed type is "Literal[1]"
+        reveal_type(m[1][0])  # N: Revealed type is "Literal[True]"
+        reveal_type(m[-1][0])  # N: Revealed type is "Literal[True]"
+[builtins fixtures/tuple.pyi]
+
+[case testMatchSubjectInferenceMapping]
+from typing import Any
+m: Any
+
+match m:
+    case {"key": 1}:
+        reveal_type(m["key"])  # N: Revealed type is "Literal[1]"
+
+[case testMatchSubjectInferenceClass]
+from typing import Final
+
+class A:
+    __match_args__: Final = ("a", "b")
+    a: str | None
+    b: int | None
+
+m: A
+
+match m:
+    case A("Hello", 2):
+        reveal_type(m.a)  # N: Revealed type is "Literal['Hello']"
+        reveal_type(m.b)  # N: Revealed type is "Literal[2]"
+    case A(a="Hello", b=2):
+        reveal_type(m.a)  # N: Revealed type is "Literal['Hello']"
+        reveal_type(m.b)  # N: Revealed type is "Literal[2]"
+    case A(a=str()) | A(a=None):
+        reveal_type(m.a)  # N: Revealed type is "Union[builtins.str, None]"
+    case object(some_attr=str()):
+        reveal_type(m.some_attr)  # N: Revealed type is "builtins.str"
+[builtins fixtures/tuple.pyi]
+
+[case testMatchSubjectInferenceOR]
+m: object
+
+match m:
+    case [1, 2, 3] | [8, 9]:
+        reveal_type(m[0])  # N: Revealed type is "Union[Literal[1], Literal[8]]"
+        reveal_type(m[1])  # N: Revealed type is "Union[Literal[2], Literal[9]]"
+        reveal_type(m[2])  # N: Revealed type is "Literal[3]"
+
+[case testMatchSubjectNested]
+from typing import Any
+class A:
+    a: str | None
+    b: int | None
+
+m: Any
+
+match m:
+    case {"key": [0, A(a="Hello")]}:
+        reveal_type(m)  # N: Revealed type is "Any"
+        reveal_type(m["key"])  # N: Revealed type is "Any"
+        reveal_type(m["key"][0])  # N: Revealed type is "Literal[0]"
+        reveal_type(m["key"][1])  # N: Revealed type is "__main__.A"
+        reveal_type(m["key"][1].a)  # N: Revealed type is "Literal['Hello']"
+    case [0, {"key": 2}]:
+        reveal_type(m[1])  # N: Revealed type is "Any"
+        reveal_type(m[1]["key"])  # N: Revealed type is "Literal[2]"
+    case object(a=[A(a="Hello") | A(a="World")]):
+        reveal_type(m.a)  # N: Revealed type is "Any"
+        reveal_type(m.a[0])  # N: Revealed type is "__main__.A"
+        reveal_type(m.a[0].a)  # N: Revealed type is "Union[Literal['Hello'], Literal['World']]"
