Improve for loop index variable type narrowing (#18014)

Preserve the literal type of index expressions a bit longer (until the
next assignment) to support TypedDict lookups.

```py
from typing import TypedDict

class X(TypedDict):
    hourly: int
    daily: int

def func(x: X) -> None:
    for var in ("hourly", "daily"):
        print(x[var])
```

Closes #9230

mypy/checker.py

@@ -3175,6 +3175,14 @@ def check_assignment(
                     # Don't use type binder for definitions of special forms, like named tuples.
                     if not (isinstance(lvalue, NameExpr) and lvalue.is_special_form):
                         self.binder.assign_type(lvalue, rvalue_type, lvalue_type, False)
+                        if (
+                            isinstance(lvalue, NameExpr)
+                            and isinstance(lvalue.node, Var)
+                            and lvalue.node.is_inferred
+                            and lvalue.node.is_index_var
+                            and lvalue_type is not None
+                        ):
+                            lvalue.node.type = remove_instance_last_known_values(lvalue_type)
 
             elif index_lvalue:
                 self.check_indexed_assignment(index_lvalue, rvalue, lvalue)
@@ -3184,6 +3192,7 @@ def check_assignment(
                 rvalue_type = self.expr_checker.accept(rvalue, type_context=type_context)
                 if not (
                     inferred.is_final
+                    or inferred.is_index_var
                     or (isinstance(lvalue, NameExpr) and lvalue.name == "__match_args__")
                 ):
                     rvalue_type = remove_instance_last_known_values(rvalue_type)

mypy/nodes.py

@@ -969,6 +969,7 @@ def is_dynamic(self) -> bool:
     "is_classvar",
     "is_abstract_var",
     "is_final",
+    "is_index_var",
     "final_unset_in_class",
     "final_set_in_init",
     "explicit_self_type",
@@ -1005,6 +1006,7 @@ class Var(SymbolNode):
         "is_classvar",
         "is_abstract_var",
         "is_final",
+        "is_index_var",
         "final_unset_in_class",
         "final_set_in_init",
         "is_suppressed_import",
@@ -1039,6 +1041,7 @@ def __init__(self, name: str, type: mypy.types.Type | None = None) -> None:
         self.is_settable_property = False
         self.is_classvar = False
         self.is_abstract_var = False
+        self.is_index_var = False
         # Set to true when this variable refers to a module we were unable to
         # parse for some reason (eg a silenced module)
         self.is_suppressed_import = False

mypy/semanal.py

@@ -4225,6 +4225,7 @@ def analyze_lvalue(
         is_final: bool = False,
         escape_comprehensions: bool = False,
         has_explicit_value: bool = False,
+        is_index_var: bool = False,
     ) -> None:
         """Analyze an lvalue or assignment target.
 
@@ -4235,6 +4236,7 @@ def analyze_lvalue(
             escape_comprehensions: If we are inside a comprehension, set the variable
                 in the enclosing scope instead. This implements
                 https://www.python.org/dev/peps/pep-0572/#scope-of-the-target
+            is_index_var: If lval is the index variable in a for loop
         """
         if escape_comprehensions:
             assert isinstance(lval, NameExpr), "assignment expression target must be NameExpr"
@@ -4245,6 +4247,7 @@ def analyze_lvalue(
                 is_final,
                 escape_comprehensions,
                 has_explicit_value=has_explicit_value,
+                is_index_var=is_index_var,
             )
         elif isinstance(lval, MemberExpr):
             self.analyze_member_lvalue(lval, explicit_type, is_final, has_explicit_value)
@@ -4271,6 +4274,7 @@ def analyze_name_lvalue(
         is_final: bool,
         escape_comprehensions: bool,
         has_explicit_value: bool,
+        is_index_var: bool,
     ) -> None:
         """Analyze an lvalue that targets a name expression.
 
@@ -4309,7 +4313,9 @@ def analyze_name_lvalue(
 
         if (not existing or isinstance(existing.node, PlaceholderNode)) and not outer:
             # Define new variable.
-            var = self.make_name_lvalue_var(lvalue, kind, not explicit_type, has_explicit_value)
+            var = self.make_name_lvalue_var(
+                lvalue, kind, not explicit_type, has_explicit_value, is_index_var
+            )
             added = self.add_symbol(name, var, lvalue, escape_comprehensions=escape_comprehensions)
             # Only bind expression if we successfully added name to symbol table.
             if added:
@@ -4361,7 +4367,12 @@ def is_alias_for_final_name(self, name: str) -> bool:
             return existing is not None and is_final_node(existing.node)
 
     def make_name_lvalue_var(
-        self, lvalue: NameExpr, kind: int, inferred: bool, has_explicit_value: bool
+        self,
+        lvalue: NameExpr,
+        kind: int,
+        inferred: bool,
+        has_explicit_value: bool,
+        is_index_var: bool,
     ) -> Var:
         """Return a Var node for an lvalue that is a name expression."""
         name = lvalue.name
@@ -4380,6 +4391,7 @@ def make_name_lvalue_var(
             v._fullname = name
         v.is_ready = False  # Type not inferred yet
         v.has_explicit_value = has_explicit_value
+        v.is_index_var = is_index_var
         return v
 
     def make_name_lvalue_point_to_existing_def(
@@ -5290,7 +5302,7 @@ def visit_for_stmt(self, s: ForStmt) -> None:
         s.expr.accept(self)
 
         # Bind index variables and check if they define new names.
-        self.analyze_lvalue(s.index, explicit_type=s.index_type is not None)
+        self.analyze_lvalue(s.index, explicit_type=s.index_type is not None, is_index_var=True)
         if s.index_type:
             if self.is_classvar(s.index_type):
                 self.fail_invalid_classvar(s.index)

test-data/unit/check-inference.test

@@ -1238,6 +1238,35 @@ class B: pass
 [builtins fixtures/for.pyi]
 [out]
 
+[case testForStatementIndexNarrowing]
+from typing_extensions import TypedDict
+
+class X(TypedDict):
+    hourly: int
+    daily: int
+
+x: X
+for a in ("hourly", "daily"):
+    reveal_type(a)  # N: Revealed type is "Union[Literal['hourly']?, Literal['daily']?]"
+    reveal_type(x[a])  # N: Revealed type is "builtins.int"
+    reveal_type(a.upper())  # N: Revealed type is "builtins.str"
+    c = a
+    reveal_type(c)  # N: Revealed type is "builtins.str"
+    a = "monthly"
+    reveal_type(a)  # N: Revealed type is "builtins.str"
+    a = "yearly"
+    reveal_type(a)  # N: Revealed type is "builtins.str"
+    a = 1  # E: Incompatible types in assignment (expression has type "int", variable has type "str")
+    reveal_type(a)  # N: Revealed type is "builtins.str"
+    d = a
+    reveal_type(d)  # N: Revealed type is "builtins.str"
+
+b: str
+for b in ("hourly", "daily"):
+    reveal_type(b)  # N: Revealed type is "builtins.str"
+    reveal_type(b.upper())  # N: Revealed type is "builtins.str"
+[builtins fixtures/for.pyi]
+
 
 -- Regression tests
 -- ----------------

test-data/unit/fixtures/for.pyi

@@ -12,9 +12,11 @@ class type: pass
 class tuple(Generic[t]):
     def __iter__(self) -> Iterator[t]: pass
 class function: pass
+class ellipsis: pass
 class bool: pass
 class int: pass # for convenience
-class str: pass # for convenience
+class str:  # for convenience
+    def upper(self) -> str: ...
 
 class list(Iterable[t], Generic[t]):
     def __iter__(self) -> Iterator[t]: pass