Inferred type is invalid

InvalidInference.hs

{-# LANGUAGE AllowAmbiguousTypes, MultiParamTypeClasses, NoMonomorphismRestriction #-}

module InvalidInference where

class Foo s t where
  foo :: t

-- When commented, GHCi infers this type; when uncommented, it yields a type error:
-- bar :: Foo s t => t
bar = foo

{-
Here's the error in question:

InvalidInference.hs:10:7: error:
    • Could not deduce (Foo s0 t) arising from a use of ‘foo’
      from the context: Foo s t
        bound by the type signature for:
                   bar :: forall s t. Foo s t => t
        at InvalidInference.hs:9:1-19
      The type variable ‘s0’ is ambiguous
      Relevant bindings include bar :: t (bound at InvalidInference.hs:10:1)
    • In the expression: foo
      In an equation for ‘bar’: bar = foo
   |
10 | bar = foo
   |       ^^^
Failed, no modules loaded.
-}