Regression in error message when attempting to let bind an existentially quantified type

When attempting to compile the following (invalid) program:

{-# LANGUAGE ExistentialQuantification #-}

data Foo = forall a. Foo a

main :: IO ()
main =
    let Foo x = Foo 1 in
    return ()

GHC 8.6.2.0 (and 8.6.1.0, 8.4.1.0) gives the following complicated error message

Test.hs:7:13: error:
    • Couldn't match expected type ‘p’ with actual type ‘a’
        because type variable ‘a’ would escape its scope
      This (rigid, skolem) type variable is bound by
        a pattern with constructor: Foo :: forall a. a -> Foo,
        in a pattern binding
        at Test.hs:7:9-13
    • In the pattern: Foo x
      In a pattern binding: Foo x = Foo 1
      In the expression: let Foo x = Foo 1 in return ()
  |
7 |     let Foo x = Foo 1 in
  |   

GHC 7.10.1.2 gave a much more helpful and direct error message

Test.hs:7:9:
    My brain just exploded
    I can't handle pattern bindings for existential or GADT data constructors.
    Instead, use a case-expression, or do-notation, to unpack the constructor.
    In the pattern: Foo x
    In a pattern binding: Foo x = Foo 1
    In the expression: let Foo x = Foo 1 in return ()

Trac metadata

Trac field	Value
Version	8.4.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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