Unexpected match with disjunction

[gafter]

One would expect the following program to behave as described in the comments. Instead, it prints true four times.

using Match

function f(x)
    @match x begin
        ((1|y), y) => true
        _ => false
    end
end

println(f((1, 1))) # true: matches (y, y)
println(f((1, 2))) # true: matches (1, y)
println(f((2, 1))) # false
println(f((2, 2))) # true: matches (y, y)

At the very least we should produce a warning for the third println, rather than silently behaving in an unexpected way.

[gafter]

In case it isn't clear what is happening, there is no link between the two y variables in the pattern ((1|y), y) because 1|y doesn't bind the variable y.

A variable pattern (like y) matches anything and binds the variable to the input value when the variable is not previously bound in the pattern. Since the disjunctive pattern (1,y) does not bind y on each disjunct, y is not bound by 1|y.

Since it is unbound where each y appears, the second y matches anything.