Issue 3586, first test case.

-- schema.json --
{
    "oneOf": [
        true,
        true
    ]
}
-- out/decode/extract --
matchN(1, [_, _])
-- test/err-string.json --
"something"
-- out/decode/testerr/err-string --
invalid value "something" (does not satisfy matchN): 2 matched, expected 1:
    generated.cue:1:1
    generated.cue:1:8
    test/err-string.json:1:1
