Exhaustiveness checking for match statements (#12267)

Closes #12010.

Mypy can now detect if a match statement covers all the possible values.
Example:
```
def f(x: int | str) -> int:
    match x:
        case str():
            return 0
        case int():
            return 1
    # Mypy knows that we can't reach here
```

Most of the work was done by @freundTech. I did various minor updates
and changes to tests.

This doesn't handle some cases properly, including these:
1. We don't recognize that `match [*args]` fully covers a list type
2. Fake intersections don't work quite right (some tests are skipped)
3. We assume enums don't have custom `__eq__` methods

Co-authored-by: Adrian Freund <adrian@freund.io>

mypy/checker.py

@@ -4089,36 +4089,57 @@ def visit_match_stmt(self, s: MatchStmt) -> None:
             if isinstance(subject_type, DeletedType):
                 self.msg.deleted_as_rvalue(subject_type, s)
 
+            # We infer types of patterns twice. The first pass is used
+            # to infer the types of capture variables. The type of a
+            # 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]
-
             type_maps: List[TypeMap] = [t.captures for t in pattern_types]
-            self.infer_variable_types_from_type_maps(type_maps)
+            inferred_types = self.infer_variable_types_from_type_maps(type_maps)
 
-            for pattern_type, g, b in zip(pattern_types, s.guards, s.bodies):
+            # The second pass narrows down the types and type checks bodies.
+            for p, g, b in zip(s.patterns, s.guards, s.bodies):
+                current_subject_type = self.expr_checker.narrow_type_from_binder(s.subject,
+                                                                                 subject_type)
+                pattern_type = self.pattern_checker.accept(p, current_subject_type)
                 with self.binder.frame_context(can_skip=True, fall_through=2):
                     if b.is_unreachable or isinstance(get_proper_type(pattern_type.type),
                                                       UninhabitedType):
                         self.push_type_map(None)
+                        else_map: TypeMap = {}
                     else:
-                        self.binder.put(s.subject, pattern_type.type)
+                        pattern_map, else_map = conditional_types_to_typemaps(
+                            s.subject,
+                            pattern_type.type,
+                            pattern_type.rest_type
+                        )
+                        self.remove_capture_conflicts(pattern_type.captures,
+                                                      inferred_types)
+                        self.push_type_map(pattern_map)
                         self.push_type_map(pattern_type.captures)
                     if g is not None:
-                        gt = get_proper_type(self.expr_checker.accept(g))
+                        with self.binder.frame_context(can_skip=True, fall_through=3):
+                            gt = get_proper_type(self.expr_checker.accept(g))
 
-                        if isinstance(gt, DeletedType):
-                            self.msg.deleted_as_rvalue(gt, s)
+                            if isinstance(gt, DeletedType):
+                                self.msg.deleted_as_rvalue(gt, s)
 
-                        if_map, _ = self.find_isinstance_check(g)
+                            guard_map, guard_else_map = self.find_isinstance_check(g)
+                            else_map = or_conditional_maps(else_map, guard_else_map)
 
-                        self.push_type_map(if_map)
-                    self.accept(b)
+                            self.push_type_map(guard_map)
+                            self.accept(b)
+                    else:
+                        self.accept(b)
+                self.push_type_map(else_map)
 
             # This is needed due to a quirk in frame_context. Without it types will stay narrowed
             # after the match.
             with self.binder.frame_context(can_skip=False, fall_through=2):
                 pass
 
-    def infer_variable_types_from_type_maps(self, type_maps: List[TypeMap]) -> None:
+    def infer_variable_types_from_type_maps(self, type_maps: List[TypeMap]) -> Dict[Var, Type]:
         all_captures: Dict[Var, List[Tuple[NameExpr, Type]]] = defaultdict(list)
         for tm in type_maps:
             if tm is not None:
@@ -4128,28 +4149,38 @@ def infer_variable_types_from_type_maps(self, type_maps: List[TypeMap]) -> None:
                         assert isinstance(node, Var)
                         all_captures[node].append((expr, typ))
 
+        inferred_types: Dict[Var, Type] = {}
         for var, captures in all_captures.items():
-            conflict = False
+            already_exists = False
             types: List[Type] = []
             for expr, typ in captures:
                 types.append(typ)
 
-                previous_type, _, inferred = self.check_lvalue(expr)
+                previous_type, _, _ = self.check_lvalue(expr)
                 if previous_type is not None:
-                    conflict = True
-                    self.check_subtype(typ, previous_type, expr,
-                                       msg=message_registry.INCOMPATIBLE_TYPES_IN_CAPTURE,
-                                       subtype_label="pattern captures type",
-                                       supertype_label="variable has type")
-                    for type_map in type_maps:
-                        if type_map is not None and expr in type_map:
-                            del type_map[expr]
-
-            if not conflict:
+                    already_exists = True
+                    if self.check_subtype(typ, previous_type, expr,
+                                          msg=message_registry.INCOMPATIBLE_TYPES_IN_CAPTURE,
+                                          subtype_label="pattern captures type",
+                                          supertype_label="variable has type"):
+                        inferred_types[var] = previous_type
+
+            if not already_exists:
                 new_type = UnionType.make_union(types)
                 # Infer the union type at the first occurrence
                 first_occurrence, _ = captures[0]
+                inferred_types[var] = new_type
                 self.infer_variable_type(var, first_occurrence, new_type, first_occurrence)
+        return inferred_types
+
+    def remove_capture_conflicts(self, type_map: TypeMap, inferred_types: Dict[Var, Type]) -> None:
+        if type_map:
+            for expr, typ in list(type_map.items()):
+                if isinstance(expr, NameExpr):
+                    node = expr.node
+                    assert isinstance(node, Var)
+                    if node not in inferred_types or not is_subtype(typ, inferred_types[node]):
+                        del type_map[expr]
 
     def make_fake_typeinfo(self,
                            curr_module_fullname: str,
@@ -5637,6 +5668,14 @@ def conditional_types(current_type: Type,
     None means no new information can be inferred. If default is set it is returned
     instead."""
     if proposed_type_ranges:
+        if len(proposed_type_ranges) == 1:
+            target = proposed_type_ranges[0].item
+            target = get_proper_type(target)
+            if isinstance(target, LiteralType) and (target.is_enum_literal()
+                                                    or isinstance(target.value, bool)):
+                enum_name = target.fallback.type.fullname
+                current_type = try_expanding_sum_type_to_union(current_type,
+                                                               enum_name)
         proposed_items = [type_range.item for type_range in proposed_type_ranges]
         proposed_type = make_simplified_union(proposed_items)
         if isinstance(proposed_type, AnyType):

mypy/checkpattern.py

@@ -1,4 +1,5 @@
 """Pattern checker. This file is conceptually part of TypeChecker."""
+
 from collections import defaultdict
 from typing import List, Optional, Tuple, Dict, NamedTuple, Set, Union
 from typing_extensions import Final
@@ -19,7 +20,8 @@
 )
 from mypy.plugin import Plugin
 from mypy.subtypes import is_subtype
-from mypy.typeops import try_getting_str_literals_from_type, make_simplified_union
+from mypy.typeops import try_getting_str_literals_from_type, make_simplified_union, \
+    coerce_to_literal
 from mypy.types import (
     ProperType, AnyType, TypeOfAny, Instance, Type, UninhabitedType, get_proper_type,
     TypedDictType, TupleType, NoneType, UnionType
@@ -55,7 +57,7 @@
     'PatternType',
     [
         ('type', Type),  # The type the match subject can be narrowed to
-        ('rest_type', Type),  # For exhaustiveness checking. Not used yet
+        ('rest_type', Type),  # The remaining type if the pattern didn't match
         ('captures', Dict[Expression, Type]),  # The variables captured by the pattern
     ])
 
@@ -177,6 +179,7 @@ def visit_or_pattern(self, o: OrPattern) -> PatternType:
     def visit_value_pattern(self, o: ValuePattern) -> PatternType:
         current_type = self.type_context[-1]
         typ = self.chk.expr_checker.accept(o.expr)
+        typ = coerce_to_literal(typ)
         narrowed_type, rest_type = self.chk.conditional_types_with_intersection(
             current_type,
             [get_type_range(typ)],
@@ -259,6 +262,9 @@ def visit_sequence_pattern(self, o: SequencePattern) -> PatternType:
         new_inner_types = self.expand_starred_pattern_types(contracted_new_inner_types,
                                                             star_position,
                                                             len(inner_types))
+        rest_inner_types = self.expand_starred_pattern_types(contracted_rest_inner_types,
+                                                             star_position,
+                                                             len(inner_types))
 
         #
         # Calculate new type
@@ -287,15 +293,20 @@ def visit_sequence_pattern(self, o: SequencePattern) -> PatternType:
 
             if all(is_uninhabited(typ) for typ in inner_rest_types):
                 # All subpatterns always match, so we can apply negative narrowing
-                new_type, rest_type = self.chk.conditional_types_with_intersection(
-                    current_type, [get_type_range(new_type)], o, default=current_type
-                )
+                rest_type = TupleType(rest_inner_types, current_type.partial_fallback)
         else:
             new_inner_type = UninhabitedType()
             for typ in new_inner_types:
                 new_inner_type = join_types(new_inner_type, typ)
             new_type = self.construct_sequence_child(current_type, new_inner_type)
-            if not is_subtype(new_type, current_type):
+            if is_subtype(new_type, current_type):
+                new_type, _ = self.chk.conditional_types_with_intersection(
+                    current_type,
+                    [get_type_range(new_type)],
+                    o,
+                    default=current_type
+                )
+            else:
                 new_type = current_type
         return PatternType(new_type, rest_type, captures)
 
@@ -344,8 +355,7 @@ def expand_starred_pattern_types(self,
                                      star_pos: Optional[int],
                                      num_types: int
                                      ) -> List[Type]:
-        """
-        Undoes the contraction done by contract_starred_pattern_types.
+        """Undoes the contraction done by contract_starred_pattern_types.
 
         For example if the sequence pattern is [a, *b, c] and types [bool, int, str] are extended
         to lenght 4 the result is [bool, int, int, str].
@@ -639,6 +649,13 @@ def construct_sequence_child(self, outer_type: Type, inner_type: Type) -> Type:
         For example:
         construct_sequence_child(List[int], str) = List[str]
         """
+        proper_type = get_proper_type(outer_type)
+        if isinstance(proper_type, UnionType):
+            types = [
+                self.construct_sequence_child(item, inner_type) for item in proper_type.items
+                if self.can_match_sequence(get_proper_type(item))
+            ]
+            return make_simplified_union(types)
         sequence = self.chk.named_generic_type("typing.Sequence", [inner_type])
         if is_subtype(outer_type, self.chk.named_type("typing.Sequence")):
             proper_type = get_proper_type(outer_type)
@@ -676,6 +693,11 @@ def get_var(expr: Expression) -> Var:
 
 
 def get_type_range(typ: Type) -> 'mypy.checker.TypeRange':
+    typ = get_proper_type(typ)
+    if (isinstance(typ, Instance)
+            and typ.last_known_value
+            and isinstance(typ.last_known_value.value, bool)):
+        typ = typ.last_known_value
     return mypy.checker.TypeRange(typ, is_upper_bound=False)
 
 

mypy/patterns.py

@@ -21,6 +21,7 @@ def accept(self, visitor: PatternVisitor[T]) -> T:
 
 
 class AsPattern(Pattern):
+    """The pattern <pattern> as <name>"""
     # The python ast, and therefore also our ast merges capture, wildcard and as patterns into one
     # for easier handling.
     # If pattern is None this is a capture pattern. If name and pattern are both none this is a
@@ -39,6 +40,7 @@ def accept(self, visitor: PatternVisitor[T]) -> T:
 
 
 class OrPattern(Pattern):
+    """The pattern <pattern> | <pattern> | ..."""
     patterns: List[Pattern]
 
     def __init__(self, patterns: List[Pattern]) -> None:
@@ -50,6 +52,7 @@ def accept(self, visitor: PatternVisitor[T]) -> T:
 
 
 class ValuePattern(Pattern):
+    """The pattern x.y (or x.y.z, ...)"""
     expr: Expression
 
     def __init__(self, expr: Expression):
@@ -73,6 +76,7 @@ def accept(self, visitor: PatternVisitor[T]) -> T:
 
 
 class SequencePattern(Pattern):
+    """The pattern [<pattern>, ...]"""
     patterns: List[Pattern]
 
     def __init__(self, patterns: List[Pattern]):
@@ -114,6 +118,7 @@ def accept(self, visitor: PatternVisitor[T]) -> T:
 
 
 class ClassPattern(Pattern):
+    """The pattern Cls(...)"""
     class_ref: RefExpr
     positionals: List[Pattern]
     keyword_keys: List[str]