Compiler crash and other oddities when using RecordWildCards with record accessors not imported

GHC exhibits an irrefutable pattern failure when attempting to compile or interpret a module that uses the RecordWildCards extension to bind a record's field names when its field accessors are not in scope.

Error:

*** Exception: compiler/rename/RnPat.lhs:535:39-79: Irrefutable pattern failed for pattern gres@(gre : _)

Examples:

{-# LANGUAGE RecordWildCards #-}
import Data.Tree (Tree(Node))
-- Note that Node's record accessors 'subForest' and 'rootLabel' are not imported.

Node{..} = Node () []

{-# LANGUAGE RecordWildCards #-}
import Data.Tree (Tree(Node))

foo = let rootLabel = (); subForest = [] in Node{..}

With GHC HEAD + a trivial patch for RnPat.lhs the programs compiled but other oddities appeared. With the patch, the first program does not have the names 'subForest' and 'rootLabel' in scope, and the second program causes GHCi to complain that the names 'rootLabel' and 'subForest' are unused, and that the fields 'Data.Tree.rootLabel' and 'Data.Tree.subForest' are not initialized in the constructor 'Node'.

I'm not sure what the correct behavior should be in these edge cases but thankfully I found a workaround in my program. (I was using RecordWildCards to extract the fields of Text.Parsec.Token.TokenParser on to the top level. If I don't import its record accessors I get the described error, and if I import the accessors unqualified I get ambiguous name errors, so instead I just import them qualified.)

Trac metadata

Trac field	Value
Version	7.4.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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