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It seems that the pattern match coverage checker does not correctly handle dictionary arguments, as evidenced by the following:

{-# LANGUAGE AllowAmbiguousTypes #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE ViewPatterns #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE TypeApplications #-}

{-# OPTIONS_GHC -Wincomplete-patterns #-}

import GHC.TypeNats (Nat, KnownNat)

pred1 :: forall (n :: Nat). Int -> Bool
pred1 i = i > 0

pred2 :: forall (n :: Nat). KnownNat n => Int -> Bool
pred2 i = i > 0

foo1 :: Int -> Int
foo1 (pred1 @12 -> True) = 0
foo1 (pred1 @12 -> False) = 0

foo2 :: Int -> Int
foo2 (pred2 @12 -> True) = 0
foo2 (pred2 @12 -> False) = 0

foo1 correctly does not report any pattern match warnings, however foo2 does, because of the dictionary argument:

Pattern match(es) are non-exhaustive
In an equation for ‘foo2’: Patterns not matched: _ :: Int