hs-boot files permit default methods in type class (but don't typecheck them)

The documentation claims you are not allowed to do so but the compiler does not reject it.

ezyang@sabre:~$ cat A.hs-boot
module A where
class C a where
    f :: a -> a
    f x = x
ezyang@sabre:~$ ghc-head -c A.hs-boot -fforce-recomp
ezyang@sabre:~$ ghc-head --version
The Glorious Glasgow Haskell Compilation System, version 8.1.20161218

The same problem applies to hsig files (which combust more spectacularly)

Trac metadata

Trac field	Value
Version	8.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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