Bad error message when unidirectional pattern synonym used in bidirectional pattern synonym

If I write:

{-# LANGUAGE PatternSynonyms #-}
module I where

pattern P2 x y <- (x, y)
pattern P1 x y = P2 x y

I get the error:

ezyang@sabre:~$ ghc-8.0 -c I.hs

I.hs:5:18: error:
    • non-bidirectional pattern synonym ‘P2’ used in an expression
    • In the expression: P2 x y
      In an equation for ‘$bP1’: $bP1 x y = P2 x y

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	low
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
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