Types leak out of function definition

[JSAbrahams]

Summary

In short, if we have a variable used within a function, say:

some variable x is declared and used a bunch

f(x)

We can use type inference to get the type of x even if we don't assign it a type above.
However, if we already know the type of x, then at most the check stage should check that it is a valid argument to the function.
If we instead match it directly, we can get weird situations if the function signature specifies the argument is a union.
Then the unification stage will also state that x is this union, which can lead to erroneous output:
Example

def f(x: {Int, Str}) -> Int => 10

def x := 10
f(x)

Will give the following:

function definition here

x: Union[int, str] = 10
f(x)

While we expect:

x: int = 10
f(x)