Prevent crashing when match arms use name of existing callable (#18449)

Fixes #16793. Fixes crash in #13666.

Previously mypy considered that variables in match/case patterns must be
Var's, causing a hard crash when a name of captured pattern clashes with
a name of some existing function. This PR removes such assumption about
Var and allows other nodes.

Author: sterliakov

mypy/checker.py

@@ -5402,17 +5402,21 @@ def _get_recursive_sub_patterns_map(
 
         return sub_patterns_map
 
-    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)
+    def infer_variable_types_from_type_maps(
+        self, type_maps: list[TypeMap]
+    ) -> dict[SymbolNode, Type]:
+        # Type maps may contain variables inherited from previous code which are not
+        # necessary `Var`s (e.g. a function defined earlier with the same name).
+        all_captures: dict[SymbolNode, list[tuple[NameExpr, Type]]] = defaultdict(list)
         for tm in type_maps:
             if tm is not None:
                 for expr, typ in tm.items():
                     if isinstance(expr, NameExpr):
                         node = expr.node
-                        assert isinstance(node, Var)
+                        assert node is not None
                         all_captures[node].append((expr, typ))
 
-        inferred_types: dict[Var, Type] = {}
+        inferred_types: dict[SymbolNode, Type] = {}
         for var, captures in all_captures.items():
             already_exists = False
             types: list[Type] = []
@@ -5436,16 +5440,19 @@ def infer_variable_types_from_type_maps(self, type_maps: list[TypeMap]) -> dict[
                 new_type = UnionType.make_union(types)
                 # Infer the union type at the first occurrence
                 first_occurrence, _ = captures[0]
+                # If it didn't exist before ``match``, it's a Var.
+                assert isinstance(var, Var)
                 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:
+    def remove_capture_conflicts(
+        self, type_map: TypeMap, inferred_types: dict[SymbolNode, 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]
 

test-data/unit/check-python310.test

@@ -2471,3 +2471,54 @@ def nested_in_dict(d: dict[str, Any]) -> int:
             return 0
 
 [builtins fixtures/dict.pyi]
+
+[case testMatchRebindsOuterFunctionName]
+# flags: --warn-unreachable
+from typing_extensions import Literal
+
+def x() -> tuple[Literal["test"]]: ...
+
+match x():
+    case (x,) if x == "test":  # E: Incompatible types in capture pattern (pattern captures type "Literal['test']", variable has type "Callable[[], Tuple[Literal['test']]]")
+        reveal_type(x)  # N: Revealed type is "def () -> Tuple[Literal['test']]"
+    case foo:
+        foo
+
+[builtins fixtures/dict.pyi]
+
+[case testMatchRebindsInnerFunctionName]
+# flags: --warn-unreachable
+class Some:
+    value: int | str
+    __match_args__ = ("value",)
+
+def fn1(x: Some | int | str) -> None:
+    match x:
+        case int():
+            def value():
+                return 1
+            reveal_type(value)  # N: Revealed type is "def () -> Any"
+        case str():
+            def value():
+                return 1
+            reveal_type(value)  # N: Revealed type is "def () -> Any"
+        case Some(value):  # E: Incompatible types in capture pattern (pattern captures type "Union[int, str]", variable has type "Callable[[], Any]")
+            pass
+
+def fn2(x: Some | int | str) -> None:
+    match x:
+        case int():
+            def value() -> str:
+                return ""
+            reveal_type(value)  # N: Revealed type is "def () -> builtins.str"
+        case str():
+            def value() -> int:  # E: All conditional function variants must have identical signatures \
+                                 # N: Original: \
+                                 # N:     def value() -> str \
+                                 # N: Redefinition: \
+                                 # N:     def value() -> int
+                return 1
+            reveal_type(value)  # N: Revealed type is "def () -> builtins.str"
+        case Some(value):  # E: Incompatible types in capture pattern (pattern captures type "Union[int, str]", variable has type "Callable[[], str]")
+            pass
+[builtins fixtures/dict.pyi]