Report required constraints when reporting the type of a hole

For the following code

module Test where

test :: String
test = show _h

GHC currently reports:

Found hole ‘_h’ with type: a0
Where: ‘a0’ is an ambiguous type variable
Relevant bindings include
  test :: String (bound at /tmp/Test.hs:4:1)
In the first argument of ‘show’, namely ‘_h’
In the expression: show _h
In an equation for ‘test’: test = show _h

It correctly does not report the lack of a Show _a instance as a separate type error. However, it would be useful if the report containing the type of the hole would also contain the constraints that apply to its type. Something like:

Found hole ‘_h’ with type: a0
Where: ‘a0’ is an ambiguous type variable
Applicable constraints: Show a0
Relevant bindings include
  test :: String (bound at /tmp/Test.hs:4:1)
In the first argument of ‘show’, namely ‘_h’
In the expression: show _h
In an equation for ‘test’: test = show _h

I am explicitly *not* suggesting to report a type like Show a0 => a0 for the hole, because that might mistakenly suggest that we are looking for a value of type forall a0. Show a0 => a0, which we are not. A possible alternative is to use an imaginary exists type like exists a0. Show a0 => a0 but that's probably just even more confusing.

Trac metadata

Trac field	Value
Version	7.8.3
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	low
Resolution	Unresolved
Component	Compiler (Type checker)
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