Warn about missing forall when -XScopedTypeVariables is on

I've ran into a situation where I forgot to put an explicit forall to actually make type variables scoped, resulting in something like

Could not deduce (PacketTypeId s0 t0 a0)
      arising from a use of ‘fromPacketTypeId’
    from the context (PacketTypeId s t n)

with code like

getPacketTypeId :: PacketTypeId s t n => KnxPacket d s t -> Integer
getPacketTypeId _ = fromPacketTypeId (Proxy :: Proxy s) (Proxy :: Proxy t)) :: Integer

It'd be nice if GHC printed an extra suggestion "Did you forget to include an explicit forall?" when something like that arises? Maybe even suggest ScopedTypeVariables if they're not on.

Trac metadata

Trac field	Value
Version	7.10.3
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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