Checking whether a default declaration is an instance of a defaultable typeclass is broken

In TcDefaults.hs, tcDefaults is supposed to check whether the supplied default type is an instance o Num or IsString (note the lack of extended defaults here, so this has been broken for quite some time).

Following the code we can see that simplifyDefault in TcSimplify.hs calls reportAllUnsolved and it does report the error -- with typechecker tracing on we see

Adding error:
  <interactive>:19:1: error:
      No instance for (Num []) arising from a 'default' declaration
      When checking the types in a default declaration

But typechecking never actually fails, so it returns Just () regardless and the error gets swallowed.

Trac metadata

Trac field	Value
Version	7.10.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	low
Resolution	Unresolved
Component	Compiler (Type checker)
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