can't match type Bool with (), but shouldn't have to

The following (definitely type-incorrect) file:

{-# LANGUAGE ScopedTypeVariables #-}

runEffect :: Either Bool r -> r
runEffect = undefined

run :: forall a. a
run = runEffect $ (undefined :: Either a ())

produces the following error:

test.hs:7:7:
    Couldn't match type `Bool' with `()'
    Expected type: a
      Actual type: ()
    In the expression: runEffect $ (undefined :: Either a ())
    In an equation for `run':
        run = runEffect $ (undefined :: Either a ())

This is strange because one of the two types it claims it can't unify (Bool) doesn't appear in either the expected or actual type. Note that removing the ($) gets a different error message that makes it a bit more clear what's going on:

test.hs:7:18:
    Couldn't match type `Bool' with `()'
    Expected type: Either Bool a
      Actual type: Either a ()
    In the first argument of `runEffect', namely
      `(undefined :: Either a ())'
    In the expression: runEffect (undefined :: Either a ())
    In an equation for `run':
        run = runEffect (undefined :: Either a ())

So it seems it's trying to unify Bool and () because it's unifying both a with Bool and a with (); however, the usual comments about rigid type variables aren't there, which makes even this error message a bit confusing.

Trac metadata

Trac field	Value
Version	7.6.3
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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