Recognize list of constant values as a list of str.

[elda27]

Feature
I wish mypy is recognized list of constant values as a list of literal types.

Pitch
There are example code.

from typing import Literal, Dict
values: Dict[Literal["from", "to", "cc"], str] = {}

# key should be recognized Literal["from","to", "cc"] instead of str
for key in ["from", "to", "cc"]: 
    values[key] = key # Now, this line will be error in order to recognize key as a str.

[parth-verma7]

@elda27 can I work upon this issue?

[elda27]

@parth-verma7
Thanks!
I have a idea to solve this problem on current version.
But there is verbose for me.

from typing import Literal, Dict
values: Dict[Literal["from", "to", "cc"], str] = {}

for key in ["from", "to", "cc"]: 
    key = cast(Literal["from", "to", "cc"], key) 
    values[key] = key # Now, this line will be error in order to recognize key as a str.

Can you tell me another simple way to solve this problem?

By the way, I have read some mypy codes at the first time and It was complicated for me.
There is type inference for key at the for statement.

mypy/mypy/checker.py

Lines 4388 to 4405 in efdda88

	def analyze_iterable_item_type(self, expr: Expression) -> tuple[Type, Type]:
	"""Analyse iterable expression and return iterator and iterator item types."""
	echk = self.expr_checker
	iterable = get_proper_type(echk.accept(expr))
	iterator = echk.check_method_call_by_name("__iter__", iterable, [], [], expr)[0]
	int_type = self.analyze_range_native_int_type(expr)
	if int_type:
	return iterator, int_type
	if isinstance(iterable, TupleType):
	joined: Type = UninhabitedType()
	for item in iterable.items:
	joined = join_types(joined, item)
	return iterator, joined
	else:
	# Non-tuple iterable.
	return iterator, echk.check_method_call_by_name("__next__", iterator, [], [], expr)[0]

In that case, it would be nice if the second element were "from"|"to"|"cc", but the understanding here seems complicated to me.
I guess echk.visitor is visitor pattern for type inference but I'm not sure other details.