Wildcard pattern with guard in first position breaks exhaustiveness of pattern match

[phagenlocher]

The following function has an exhaustive pattern match which is correctly identified as such:

f : [A, B], U8 -> Bool
f = \x, y ->
    when x is
        A -> Bool.true
        B -> Bool.false
        _ if y > 1 -> Bool.false

Note that the wildcard does not make this pattern exhaustive. This is already covered by the cases A and B.

When the wildcard pattern is moved in between the A and B patterns, the following code is still fine:

f : [A, B], U8 -> Bool
f = \x, y ->
    when x is
        A -> Bool.true
        _ if y > 1 -> Bool.false
        B -> Bool.false

However, when making the the wildcard pattern the first pattern of the match roc check incorrectly reports this as an error:

── UNSAFE PATTERN in src/Test.roc ──────────────────────────────────────────────

This when does not cover all the possibilities:

5│>      when x is
6│>          _ if y > 1 -> Bool.false
7│>          A -> Bool.true
8│>          B -> Bool.false

Other possibilities include:

    _    (note the lack of an if clause)

I would have to crash if I saw one of those! Add branches for them!

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1 error and 0 warnings found in 42 ms