add tests for covariant case

Author: carljm

crates/ty_python_semantic/resources/mdtest/generics/legacy/functions.md

@@ -892,11 +892,19 @@ FlatT = TypeVar("FlatT")
 def flatten(*iterables: Iterable[FlatT]) -> list[FlatT]:
     return [x for iterable in iterables for x in iterable]
 
+def flatten_covariant(*iterables: Iterable[FlatT]) -> tuple[FlatT, ...]:
+    return tuple(x for iterable in iterables for x in iterable)
+
 reveal_type(flatten("abc", (1, 2, 3)))  # revealed: list[str | int]
+# TODO: we could have `Literal["a", "b", "c"]` instead of `str` here
+reveal_type(flatten_covariant("abc", (1, 2, 3)))  # revealed: tuple[str | Literal[1, 2, 3], ...]
 
 def literal_string_case(literal_string: LiteralString):
     reveal_type(flatten(literal_string, (1, 2, 3)))  # revealed: list[str | int]
 
 reveal_type(flatten(b"abc"))  # revealed: list[int]
 reveal_type(flatten(b"abc", ("x",)))  # revealed: list[int | str]
+reveal_type(flatten_covariant(b"abc"))  # revealed: tuple[int, ...]
+# TODO: we could have `Literal[97, 98, 99]` instead of `int` here
+reveal_type(flatten_covariant(b"abc", ("x",)))  # revealed: tuple[int | Literal["x"], ...]
 ```