ExtendedDefaultRules-related regression in GHC 8.0.2

Commit https://git.haskell.org/ghc.git/commit/9a34bf1985035858ece043bf38b47b6ff4b88efb introduced a regression between GHC 8.0.1 and 8.0.2 regarding the behavior of ExtendedDefaultRules. In GHC 8.0.1, this program

module Bug where

default (Bool)

would compile without issue. On GHC 8.0.2, however, it is rejected with this error message:

$ /opt/ghc/8.0.2/bin/ghc Bug.hs
[1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

Bug.hs:3:1: error:
    • The default type ‘Bool’ is not an instance of ‘Num’
    • When checking the types in a default declaration

As a result, the shelly library now fails to compile with GHC 8.0.2 (see https://github.com/yesodweb/Shelly.hs/issues/130). A workaround is to explicitly enable ExtendedDefaultRules at the top of the module.

Richard, do you understand what's going on here? I'm not sure if this is a real regression or just GHC being more particular (admittedly, I don't understand many of the intricacies of ExtendedDefaultRules).

Trac metadata

Trac field	Value
Version	8.0.2-rc1
Type	Bug
TypeOfFailure	OtherFailure
Priority	high
Resolution	Unresolved
Component	Compiler (Type checker)
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