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If you create a value whose type has a nullary constraint (i.e.: a constraint that does not reference any of the type variables in scope) and then try to run :t on it, GHCi attempts to run instance resolution and fails before printing the type.

Expected output:

GHCi, version 8.4.3: http://www.haskell.org/ghc/  :? for help
Prelude> :set -XFlexibleContexts
Prelude> data Foo = Foo
Prelude> let x :: (Show Foo) => () ; x = ()
Prelude> :t x
x :: Show Foo => ()

Actual output:

GHCi, version 8.4.3: http://www.haskell.org/ghc/  :? for help
Prelude> :set -XFlexibleContexts
Prelude> data Foo = Foo
Prelude> let x :: (Show Foo) => () ; x = ()
Prelude> :t x

<interactive>:1:1: error:
    No instance for (Show Foo) arising from a use of ‘x’

Trac metadata

Trac field	Value
Version	8.4.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	GHCi
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