New solver: Not inferring union type on reassignments

[Ketasaja]

Luau 0.658

I expected no error, but got TypeError: Type '{ a: string, b: string }' could not be converted into 'ty'; type { a: string, b: string }[read "b"] (string) is not exactly ty[read "b"][1] (nil) when reassigning t.

The workaround is b = "" :: string?,.

--!strict
type ty = {
	b: string?,
}
local t: ty = {
	b = "",
}

t = {
	b = "", -- TypeError: Type '{ b: string }' could not be converted into 'ty'; type { b: string }[read "b"] (string) is not exactly ty[read "b"][1] (nil)
}