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In #17343 we saw that we didn't handle the pattern guard `!_ <- undefined` correctly: The `undefined` was never evaluated. Indeed, elaboration failed to insert the invisible type aruments to `undefined`. So `undefined` was trivially a normal-form and in turn never entered. The problem is that we used to infer a sigma-type for the RHS of the guard, the leading qualifiers of which will never be useful in a pattern match situation. Hence we infer a rho-type now. Fixes #17343.
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