When in strict mode in typed Luau, refining the type of a value using a simple if foo then guard doesn’t work unless foo has an explicit type annotation. Using all other method of type refinement work just fine.
As an example, the following code should be type safe, but according to the type checker, it is not.
--!strict
local foo: number? = 1
local bar = foo
if bar then
local baz: number = bar
end
However, adding a type annotation to bar causes it to become safe according to the analyzer:
--!strict
local foo: number? = 1
local bar: typeof(foo) = foo
if bar then
local baz: number = bar
end
This doesn’t change the types at all but fixes a script analysis error. This is particularly important for table types, as you can’t check against them using any of the other type refinements, so something like { }? can’t be refined otherwise.
This occurs in studio version 0.445.0.410316.